Slardar
* 
Bloodseeker's 
Rupture forces Slardar to stay still during fights, preventing Slardar from using Guardian sprint to move in and out of fights. This takes away the ability to initiate and re-initiate with Slithereen Crush's low cooldown. Due to the increased move speed from Guardian Sprint, Slardar can lose health at about 160% the rate of normal heroes while under Rupture. Bloodseeker's 
Thirst also gives him enough move speed so that he's one of the few heroes who can outrun or catch up to a Slardar during Guardian Sprint.
Dark Seer
* Heroes with fast movement abilities will suffer huge damage when afflicted by Rupture: 
Spirit Breaker, 
Earth Spirit, 
Dark Seer.
Additional:
* Rupture can deal a lot of damage to either a surged Dark Seer or his allies and prevent the target from getting into position.
Slardar
* Abaddon can cast 
Aphotic Shield on any of his allies to immediately dispel Corrosive Haze and/or the stun and the slow components of Slithereen Crush.
* All of Slardar's debuffs are also removed if Abaddon's 
Borrowed Time triggers.
Slardar
* 
Fire Spirits greatly reduces Slardar's already low starting attack speed (which Bash of the Deep relies on), making the laning phase very hard for him and discouraging Slardar from aggressively threatening lane kills.
* 
Supernova, ignoring Phoenix's lack of armor and Slardar's high damage single hits, is extremely hard for Slardar to take down on his own, especially after being hit by Fire Spirits.
* 
Icarus Dive is one of the longest distance escape spells in the game, and can help a good Phoenix escape from Slardar before he gets a 
Blink Dagger. However, the high cooldown means Phoenix is vulnerable to backstabs from any roamers after said spell is used.
* Slardar can interrupt an Icarus Dive from completing the flight with both of his stuns, however, likely leading to death if Supernova is on cooldown.
* A lot of the items a Phoenix builds throughout the game naturally counter Slardar. 
Shiva's Guard, 
Heaven's Halberd and 
Eul's Scepter of Divinity all stall Slardar's normal attacks in some way, while 
Spirit Vessel's percentage-based damage is good against his innate durability.
Slardar
* Slardar has high priority on building moblity items such as Blink Dagger rather than tank items. Zeus can easily deal large amount of damage to him if not caught out of position first.
Dark Seer
* 
X Marks the Spot prevents Dark Seer or his allies from escaping with Surge.
Dark Seer
* 
Flame Guard makes Dark Seer not deal any damage with his skills to Ember Spirit.
Dark Seer
* 
Spell Steal will easily steal all abilities from Dark Seer.
Additional:
* Spell Steal can perfect from Rubick's steal all abilities from Dark Seer. Rubick's always purchase 
Aghanim's Scepter can two stolens abilities from Dark Seer's and waiting for seconds for second stolen abilities (example: combinated Vacuum and Wall of Replica) and many different combination.
* A good positoning from Rubick's Spell Steal waiting steal abilities from Dark Seer 
Wall of Replica and illusion from Wall of Replica Dark Seer's can fighting with effect from outcome steal Wall of Replica from Rubick's.
* Combinated Rubick's steal 
Surge and Blink Dagger is very important catch from Dark Seer's and enemies.
* Rubick's 
Telekinesis can lifted up Dark Seer or enemy heroes and throw to Wall of Replica from Rubick's steal.
Dark Seer
* The constant dispel from Demonic Purge will make any attempt to save the victim or himself by Surge become futile. For what it's worth, Demonic Purge will also dispel 
Ion Shell.
Slardar
* Illusion heroes like the aforementioned 
Phantom Lancer or 
Naga Siren, or heroes that tend to buy 
Manta Style, like 
Luna or 
Juggernaut, will give Slardar a hard time, as his damage is single-targeted.
Dark Seer
* Dark Seer relies on Surge for most of his mobility, so he's not very vulnerable to slows, but much more to root effects and walls or pseudo-walls such as 
Disruptor's 
Kinetic Field or 
Slark's 
Pounce.
Slardar
* 
Tidehunter's 
Kraken Shell dispels Corrosive Haze and Bash of the Deep's stun if enough damage is taken. 
Anchor Smash and Kraken Shell's damage block also reduces Slardar's damage.
Dark Seer
* Dark Seer's only disable is Vacuum. Though he starts off with high armor and health, he'll eventually have a hard time dealing with high mobility heroes such as 
Weaver or 
Queen of Pain.
Dark Seer
* In general, heroes who are melee with no way to fight back can be very easily poked by Monkey King to gain his Jingu Mastery stacks: Dark Seer, 
Riki, 
Rubick.
Dark Seer
* Strong laners are usually at a disadvantage because of how well Sven can jungle. Heroes such as Dark Seer. 
Underlord and 
Legion Commander may find that later Sven takes care of them very quickly after some jungle farming.
Slardar
* Illusion heroes like the aforementioned Phantom Lancer or Naga Siren, or heroes that tend to buy Manta Style, like Luna or Juggernaut, will give Slardar a hard time, as his damage is single-targeted.
Slardar
* Phantom Lancer's 
Doppelganger has a low cooldown and applies a basic dispel on PL, making it hard to keep Corrosive Haze on him.
* Phantom Lancer usually purchases Manta Style which provides another basic dispel to remove Corrosive Haze with.
* Besides Slithereen Crush, Slardar does not have another way to deal with illusions and has a low mana pool, which allows Phantom Lancer with 
Diffusal Blade to quickly build up illusions while draining Slardar's mana.
* 
Phantom Rush outruns Guardian Sprint, forcing Slardar to Blink away or use Slithereen Crush defensively to avoid being killed.
Dark Seer
* Heroes that rely on illusions typically spawn very strong illusions inside Dark Seer Wall of Replica that he can use against them or their team: Phantom Lancer, 
Chaos Knight, Naga Siren, 
Terrorblade.
Additional:
* Ion Shell helps taking down illusions quickly, making it easy to identify the real Phantom Lancer.
* Surge allows Dark Seer and his teammates to run away from Phantom Lancer, who relies on his Phantom Rush to chase enemies.
Slardar
* 
Corrosive Haze will reveal Treant Protector even when he is under the effects of Nature's Guise.
Dark Seer
* 
Living Armor can be destroyed quickly with Ion Shell.
Slardar
* While Slardar has Guardian's Sprint activated, 
Powershot cannot slow him and is easier to avoid.
* Slithereen Crush and Bash of the Deep help Slardar put Windranger in place, even when she uses 
Windrun to escape.
* Corrosive Haze from Slardar is very effective against Windranger's already low armor. This becomes even worse if she has Aghanim's Scepter-upgraded Windrun, because it also reveals her position and allowing his allies to use their abilities on Windranger.
* Note that the invisibility provided by Aghanim's Scepter-upgraded Windrun is not broken upon attacking, casting abilities or using items, so it's vital to cast Corrosive Haze beforehand.
Dark Seer
* With 
Normal Punch from Aghanim's Scepter, Dark Seer can gain an advantage against heroes who rely on evasion as Normal Punch has true strike: 
Phantom Assassin, 
Windranger, 
Hoodwink.
Slardar
* 
Slardar can attack through Plague Wards and gain 
Bash of the Deep charges, and 
Templar Assassin can spill 
Psi Blades through Plague Wards too.
Dark Seer
* Venomancer is a low health hero and low range hero who is easy to harass with Ion Shell on a creep in the early game.
* Venomancer depends entirely on all his slows to keep distance or chase, but since Surge ignores slows, it is nearly impossible for Venomancer to hunt or run away from Dark Seer or his surged teammates.
Slardar
* Corrosive Haze hinders Nyx Assassin's ganking attempts with 
Vendetta, as well as reducing his armor and make him very vulnerable due to his low health pool.
* 
Guardian Sprint allows Slardar to catch up with Nyx Asssassin and finish him off if he has vision over him.
Dark Seer
* Heroes who rely on damage over time as Spiked Carapace can block and reflect only one instance of damage: 
Jakiro, 
Viper, 
Venomancer, 
Huskar, 
Ember Spirit, Dark Seer, 
Sand King, 
Doom, 
Dawnbreaker. However, this also means they are easily stunned.
Slardar
* Heroes who can provide True Sight are always credible threats: Slardar, 
Bounty Hunter, especially 
Zeus since his 
Lightning Bolt provides True Sight without the need to detect her beforehand to apply the debuff. So even a late-coming Zeus can easily scout a large area to reveal her, and she cannot purge herself from his sight with the dispel talent.
Dark Seer
* Ion Shell destroys 
Refraction's protection stacks in short time.
* Surge ignores the massive slow from 
Psionic Trap.
Slardar
* Heroes who can provide True Sight or semi-reveal enemy heroes as Nature's Prophet is prone to buying 
Shadow Blade: Bounty Hunter, Slardar, 
Spectre, Zeus, Spirit Breaker.
Dark Seer
* Heroes that rely on summons will suffer against Ion Shell, which kills them very quickly: 
Beastmaster, 
Broodmother, 
Enigma, 
Nature's Prophet, 
Visage, 
Warlock, 
Wraith King.
Slardar
* Slardar's 
Slithereen Crush allows him to stun Spectre and illusions, and Corrosive Haze prevents her from juking with 
Spectral Dagger.
Dark Seer
* Dark Seer can pull together Spectre's illusions with 
Vacuum, and Wall of Replica creates an illusion of Spectre that deals massive damage.
Slardar
* Heroes with high physical damage can damage Lifestealer more than he can heal: 
Bristleback, 
Clinkz, Luna,
Shadow Fiend, Slardar, Terrorblade
Dark Seer
* Heroes with escape abilities: 
Anti-Mage Dark Seer Slark 
Storm Spirit Windranger Weaver can kite Lifestealer due to him having no lockdown.
Slardar
* Heroes with armor reducing abilities greatly hinder Visage's defense like Slardar and 
Dazzle.
Dark Seer
* Heroes that rely on summons will suffer against Ion Shell, which kills them very quickly: Beastmaster, Broodmother, Enigma, Nature's Prophet, Visage, Warlock, Wraith King.
Dark Seer
* Dark Seer's Vacuum and Ion Shell can clear a wave of Chen's Creeps effectively.
* Surge applies haste movement and negates the slow from 
Penitence, allowing Dark Seer to easily escape from Chen and his creeps.
* Dark Seer's full combo from Dark Seer's Vacuum, Wall of Replica and Normal Punch from Aghanim's Scepter can often kill Chen or any of his allies before he can save them with 
Hand of God.
Additional:
* Dark Seer's Vacuum and Ion Shell can clear a wave of Chen's Creeps effectively.
* Surge applies haste movement and negates the slow from Penitence, allowing Dark Seer to easily escape from Chen and his creeps.
* Dark Seer's full combo from Dark Seer's Vacuum, Wall of Replica and Normal Punch from Aghanim's Scepter can often kill Chen or any of his allies before he can save them with Hand of God.
Slardar
* Shadow Fiend, a normal Shadow Blade builder often relies on its invisibility to escape fights. Corrosive Haze reveals Shadow Fiend and also weakens his already low armor.
Slardar
* Corrosive Haze works much like 
Track, giving the enemy team vision over Riki. While it doesn't give a movement speed or gold bonus, it instead can melt Riki's armor and Slardar can use Guardian Sprint to catch up to him.
* Slardar's 
Aghanim's Shard, causes Slithereen Crush to apply Corrosive Haze even if Slardar does not have vision over him.
Additional:
* Corrosive Haze makes Riki completely vulnerable to Slardar and his allies, as Riki has no easy way of dispelling it.
* Slardar's physical damage with Slithereen Crush and Bash of the Deep amplified by Corrosive Haze can easily kill Riki.
Slardar
* Guardian Sprint allows Slardar to outrun Bristleback, even when slowed by Viscous Nasal Goo.
* Corrosive Haze's armor reduction makes Bristleback much less tanky.
Slardar
* The brutal physical damage of Slithereen Crush and Bash of the Deep can quickly remove Void Spirit's shield and stun him for a while.
* Corrosive Haze provides vision of Void Spirit while he is running away and Guardian Sprint allows Slardar to quickly close the gap
Slardar
* Slardar can reveal Clinkz with Corrosive Haze and make him easy to kill, and his Bash of the Deep also hits hard against Clinkz's naturally low health pool.
* Slardar's Guardian Sprint can catch up with Clinkz's Skeleton Walk.
* The stun and slow from Slardar's Slithereen Crush can hold Clinkz in place.
Additional:
* Clinkz survivability depends entirely on 
Skeleton Walk. Corrosive Haze not only makes its invisibility useless, but also makes Clinkz even more vulnerable to physical damage.
Slardar
* Corrosive Haze followed by Slithereen Crush takes advantage of Alchemist's main weakness, being physical damage before he has the change to improve his surviability with items.
* Bash of the Deep will deal lots of damage during the laning phase.
Slardar
* Corrosive Haze provides True Sight on its target, exposing Bounty Hunter's position regardless of Shadow Walk.
* Guardian Sprint gives a very high movement speed bonus, allowing him to chase Bounty Hunter even if Slardar is tracked.
Additional:
* Bounty Hunter's 
Shadow Walk is rendered useless with Corrosive Haze and makes Bounty Hunter an easy target for Slardar and his team.
Slardar
* Slardar is one of the few meele heroes who can lane against Ursa in the early game, out harassing him with Bash of the Deep, and running away from a bad engagement with Guardian Sprint
* Corrosize Haze can force Ursa to cast 
Enrage defensively instead of offensively
* With Corrosize Haze, Slardar's team can quickly dispatch of Roshan, denying one of Ursa's greatest advantages
Slardar
* With armor items like 
Solar Crest or 
Assault Cuirass combined with Bash of the Deep, Slardar can easily tank the 
Duel, If not dueled himself, he can drop Legion Commander's armor during duel and/or buff his teammate and save them, leaving her only the choice of dueling supports which can also be saved with fast Blink into Slithereen Crush.
Dark Seer
* Ion Shell zones out Broodmother in the laning phase, and can kill her spiderlings if used on one Spiderling in a group.
* Vacuum can pull Broodmother out of the treeline.
Additional:
* Ion Shell is a very powerful weapon against Broodmother's Spiderlings and makes her laning much harder.
Slardar
* Corrosive Haze and Bash of the Deep will make short work of Huskar's low armor, even if Slardar is alone.
Dark Seer
* With Normal Punch from Aghanim's Scepter, Dark Seer can gain an advantage against heroes who rely on evasion as Normal Punch has true strike: Phantom Assassin, Windranger, Hoodwink.
Dark Seer
* Heroes with haste can negate Enchant's slow: Weaver, Dark Seer, 
Lycan, 
Centaur Warrunner.
Dark Seer
* Certain heroes can mess up with Pangolier's positioning and timing (especially if he intends to use Rolling Thunder during combat): 
Vengeful Spirit, 
Keeper of the Light, Huskar, Dark Seer
Dark Seer
* With Normal Punch from Aghanim's Scepter, Dark Seer can gain an advantage against heroes who rely on evasion as Normal Punch has true strike: Phantom Assassin, Windranger, Hoodwink.
Dark Seer
* Heroes that rely on summons will suffer against Ion Shell, which kills them very quickly: Beastmaster, Broodmother, Enigma, Nature's Prophet, Visage, Warlock, Wraith King.
Dark Seer
* Heroes who do not have nor rely on passive abilities make Nethertoxin much less effective: Clinkz, 
Pugna, Nature's Prophet, Dark Seer, Lycan, 
Tusk
Dark Seer
* Dark Seer can increase the movement speed of allies with Surge, breaking Static Link.
Dark Seer
* Heroes that rely on illusions typically spawn very strong illusions inside Dark Seer Wall of Replica that he can use against them or their team: Phantom Lancer, Chaos Knight, Naga Siren, Terrorblade.
Slardar
* 
Crystal Maiden's already low armor and easily interruptible ultimate, 
Freezing Field can make Slardar her worst nightmare. Crystal Maiden also often builds 
Glimmer Cape which is revealed by Corrosive Haze.
Dark Seer
* Heroes with hastes: Dark Seer, Lycan
Dark Seer
* Dark Seer relies on Surge for most of his mobility, so he's not very vulnerable to slows, but much more to root effects and walls or pseudo-walls such as Disruptor's Kinetic Field or Slark's Pounce.
Slardar
* Slardar's level 25 talent makes Corrosive Haze undispellable, thus providing his allies vision of Slark.
Dark Seer
* Heroes with AoE abilities will hit Slark in Shadow Dance: 
Lina, Jakiro, Underlord, 
Leshrac, Dark Seer, 
Earthshaker, 
Timbersaw, Crystal Maiden.
Slardar
* Slardar can chase Dragon Knight down with Guardian Sprint and stun him with both Slithereen Crush and his passive Bash of the Deep. He can also use Corrosive Haze to whittle Dragon Knight's armor.
Dark Seer
* The haste provided by Dark Seer's Surge or a {Rune:Haste} renders Lich's slows ineffective.
Dark Seer
* Heroes that rely on summons will suffer against Ion Shell, which kills them very quickly: Beastmaster, Broodmother, Enigma, Nature's Prophet, Visage, Warlock, Wraith King.
Additional:
* Surge allows for Dark Seer and his allies to escape from Upheaval due to it ignoring any slow.
Slardar
* Bounty Hunter and Slardar both have ultimates (Corrosive Haze and Track) that reveal Invoker through Ghost Walk.
Slardar
* Heroes with spells that negate invisibility such as Bounty Hunter, Zeus and Slardar prevent her from saving her allies with Moonlight Shadow.
Dark Seer
* Heroes that rely on summons will suffer against Ion Shell, which kills them very quickly: Beastmaster, Broodmother, Enigma, Nature's Prophet, Visage, Warlock, Wraith King.
Slardar
* Slardar does not do well against multiple targets, as his only way to damage multiple targets is Slithereen Crush.
* Meepo's 
Earthbind will root Slardar, making him unable to chase/escape for the time being. Also, 
Poof's low cooldown and high damage can burst Slardar down rather quickly.
Slardar
* Heroes who can stun multiple units at once: 
Dark Willow, Spirit Breaker, Slardar, 
Invoker, and Tidehunter.
Dark Seer
* Dark Seer can take great advantage of 
Divided We Stand by putting an Ion Shell on a clone in order to harass them effectively.
* Vacuum can take a single Meepo and disable him for Dark Seer and his teammates to take out.
Slardar
* Terrorblade starts out with and gains a lot of armor from his agility growth. His illusions also make it difficult for Slardar to focus down the real Terrorblade.
Dark Seer
* Dark Seer's Vacuum and Ion Shell allows him to clear a wave of Terrorblade's illusions.
* Because Terrorblade often builds items that require 
Ultimate Orb like Manta Style and 
Eye of Skadi, Dark Seer can create a very strong illusion of Terrorblade with Wall of Replica and Normal Punch from Aghanim's Scepter.
* If Dark Seer can use Vacuum to help teammates land their AoE abilities, Terrorblade will be quickly killed before he even gets a chance to use Sunder.
Additional:
* Dark Seer's Vacuum and Ion Shell allows him to clear a wave of Terrorblade's illusions.
* Because Terrorblade often builds items that require Ultimate Orb like Manta Style and Eye of Skadi, Dark Seer can create a very strong illusion of Terrorblade with Wall of Replica and Normal Punch from Aghanim's Scepter.
* If Dark Seer can use Vacuum to help teammates land their AoE abilities, Terrorblade will be quickly killed before he even gets a chance to use Sunder.
Dark Seer
* Dark Seer's only disable is Vacuum. Though he starts off with high armor and health, he'll eventually have a hard time dealing with high mobility heroes such as Weaver or Queen of Pain.
Slardar
* Weaver relies on 
Shukuchi to escape and kite enemy heroes. Slardar's Corrosive Haze lowers armor and reveals Weaver, who is already a very frail target.
* Weaver usually builds 
Linken's Sphere, Corrosive Haze can be used to pop the spell block, additionally the level 25 undispellable talent will prevent Weaver from removing Corrosive Haze with 
Time Lapse, allowing allied heroes to use their abilities on Weaver.
Additional:
* Corrosive Haze lowers armor and reveals Weaver during Shukuchi, who is already a frail target. In addition, Weaver usually builds Linken's Sphere, and Corrosive Haze can be used to "pop" the spell block. His level 25 talent can also make Corrosive Haze undispellable by Time Lapse, which puts Weaver in an even worse situation.
* Guardian Sprint can dodge Weaver's bugs with the movement speed bonus.
Dark Seer
* Abilities that deal low damage with high amounts of instances are irrelevant against Pudge during Flesh Heap's active: 
Gyrocopter's 
Rocket Barrage and Dark Seer's Ion Shell are just a couple examples.
Slardar
* Corrosive Haze reveals Pudge's location, making it hard for him to use Meat Hook from the fog of war on Slardar's teammates or Slardar himself.
* Slithereen Crush interrupts Pudge while he is using Dismember.
* Guardian Sprint allows Slardar to avoid Meat Hook.
Additional:
* While Slardar has Guardian Sprint activated, 
Rot cannot slow him and 
Meat Hook is easier to avoid.
* Slithereen Crush allows Slardar to interrupt 
Dismember at close range (when Slardar is not a target).
* Corrosive Haze reveals Pudge's location, making it very difficult for him to land hooks from the fog.
Slardar
* Illusion heroes like the aforementioned Phantom Lancer or Naga Siren, or heroes that tend to buy Manta Style, like Luna or Juggernaut, will give Slardar a hard time, as his damage is single-targeted.
Dark Seer
* Heroes that rely on illusions typically spawn very strong illusions inside Dark Seer Wall of Replica that he can use against them or their team: Phantom Lancer, Chaos Knight, Naga Siren, Terrorblade.
Dark Seer
* Heroes without blink mechanisms or spell immunity will have a hard time positioning against 
Pit of Malice: 
Sven, 
Tiny, Wraith King, 
Dragon Knight, 
Meepo, Doom, Bounty Hunter, Dark Seer, 
Undying, 
Alchemist, Legion Commander.
Dark Seer
* Heroes with strong area-of-effect damaging and/or disabling abilities will be able to intercept Underlord and his team when they exit from Fiend's Gate: Luna's Aghanim's Scepter upgraded 
Eclipse, Dawnbreaker's 
Solar Guardian, 
Axe's 
Berserker's Call, 
Mars's 
Arena of Blood, Jakiro's 
Macropyre, 
Dual Breath and 
Ice Path, Naga Siren's 
Song of the Siren, 
Razor's 
Eye of the Storm and 
Plasma Field, Lina's 
Light Strike Array, Broodmother's 
Spinner's Snare from Aghanim's Scepter, 
Elder Titan's 
Earth Splitter and 
Echo Stomp, Alchemist's 
Unstable Concoction and 
Acid Spray, Queen of Pain's 
Sonic Wave, Dark Seer's Wall of Replica and Vacuum, Venomancer's 
Noxious Plague and 
Poison Nova from Aghanim's Scepter, 
Oracle's 
Fortune's End and 
Rain of Destiny from Aghanim's Shard, Bloodseeker's 
Blood Rite, Enigma's 
Black Hole and 
Midnight Pulse, Shadow Fiend's 
Requiem of Souls, 
Batrider's Aghanim's Scepter upgraded 
Flaming Lasso, 
Firefly, 
Flamebreak and 
Sticky Napalm, 
Ancient Apparition's 
Ice Blast, Sand King's 
Epicenter, 
Sand Storm and 
Burrowstrike, Gyrocopter's 
Call Down, Ember Spirit's 
Sleight of Fist, 
Death Prophet's 
Exorcism and 
Silence, 
Monkey King's 
Wukong's Command, Crystal Maiden's Freezing Field, 
Muerta's 
The Calling, 
Medusa's 
Stone Gaze, Zeus's 
Nimbus from Aghanim's Scepter. Dark Willow's 
Bramble Maze, 
Pangolier's 
Swashbuckle, 
Grimstroke's 
Soulbind, 
Ink Swell and 
Stroke of Fate, Earthshaker's 
Echo Slam and 
Fissure, 
Puck's 
Dream Coil and 
Waning Rift, 
Winter Wyvern's 
Winter's Curse, 
Magnus's 
Reverse Polarity.
Dark Seer
* A Sniper who is somewhat competent at positioning is hardly going to be affected by any of Dark Seer's spells thanks to 
Take Aim.
* However, an illusion of Sniper made with Wall of Replica is extremely powerful, and hard to deal with.
Slardar
* Slardar can use Guardian Sprint with Blink Dagger or Shadow Blade to get close, and Slithereen Crush will stun Sniper and slow him, making him vulnerable to Slardar's teammates.
* Corrosive Haze lowers Sniper's armor and makes Sniper's Shadow Blade useless, easily killing him with a few attacks.
Slardar
* Enigma can be a perfect counter-initiator with his ultimate. A perfectly timed Black Hole will turn the tide of any initiation, especially from heroes like; Spirit Breaker, Legion Commander, Axe, Puck, and Slardar.
Dark Seer
* Heroes that rely on summons will suffer against Ion Shell, which kills them very quickly: Beastmaster, Broodmother, Enigma, Nature's Prophet, Visage, Warlock, Wraith King.
Dark Seer
* Dark Seer's strength lies in his high-mana cost skills. So 
Mana Break can be a great nuisance for him, since it can reduce his mana and prevent him from casting his spells.
* 
Mana Void can deal a lot of damage if Dark Seer spent a lot of mana on his abilities.
Slardar
* Slardar deals entirely physical damage. He does not have to worry about Counterspell.
* Slithereen Crush and Bash of the Deep prevent Anti-Mage from using Blink, the latter even if Anti-Mage activates 
Black King Bar.
* Corrosive Haze allows allies to take down Anti-Mage before he has a chance to Blink, and maintains vision over him even if he does Blink away.
Additional:
* Slithereen Crush and Bash of the Deep deals physical damage, bypassing the magic resistance bonus from 
Counterspell
* Corrosize Haze significantly increases the damage dealt against Anti-Mage, and makes it harder for him to flee with 
Blink, although Anti-mage can block its casting with with Counterspell
Slardar
* Medusa cares less than other heroes about armor reduction, since 
Mana Shield blocks 70% of all damage without taking armor/magic resistance into consideration. This makes 60% of your Corrosive Haze ineffective until Medusa's mana is drained.
Slardar
* Other heroes who has armor and slow debuffs won't allow Medusa to disengage from combat: Tidehunter, Bristleback, Slardar.
Slardar
*
Fortune's End applies a basic dispel which can be used to dispel Corrosive Haze, or even Slardar's own Guardian Sprint.
False Promise is able to dispel Corrosive Haze and Slithereen Crush stun/slow from any allies Oracle chooses to cast it on.
Fate's Edict can also be cast on Slardar to prevent him from proccing his Bash of the Deep.
*
*
Dark Seer
*
Fortune's End dispels Ion Shell and Surge, making any gank attempt on Dark Seer in the laning phase likely to succeed.
Fate's Edict prevents all magical damage that comes from Dark Seer's abilities. 
Fortune's End dispels Ion Shell on creeps, which Dark Seer relies on to farm and zone.
Ion Shell, a skill that helps Dark Seer immensely in the laning phase, as well as 
Surge, a move-speed buff, effectively allowing Oracle to control Dark Seer, especially in the laning phase.
*
*
Additional:
* Fortune's End can dispel