From Dota 2 Wiki
- Dark Seer's strength lies in his high-mana cost skills. Mana Break Gets rid of his mana (ergo, he can neither use Surge to escape, nor any of his offensive skills to fight).
- Mana Leak is very dangerous to Dark Seer, because even if he tries to escape with Surge he will lose all of his mana and get stunned.
- Blinding Light makes landing the Vacuum + Wall of Replica combo harder.
- Oracle can remove Dark Seer's Ion Shell and/or Surge by casting Fortune's End. This makes any gank attempt on Dark Seer in the laning phase very likely to succeed.
- Fate's Edict prevents all magical damage that comes from Dark Seer's abilities.
- A Sniper who is somewhat competent at positioning will never be hit by any of Dark Seer's spells thanks to Take Aim.
- 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 psedo-walls such as Disruptor's Kinetic Field or Slark's Pounce.
- 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, either killing them or escaping from them.
- Orchid Malevolence prevents Dark Seer from using Surge and running away.
- Blade Mail can deal much damage to Dark Seer if he uses his Ion Shell on a creep.
- Diffusal Blade and Eul's Scepter of Divinity can purge off Ion Shell and Surge.
- Ion Shell is a very powerful weapon against Broodmother's Spiderlings. It makes her laning much harder.
- Heroes that rely on illusions will suffer against Ion Shell, which kills them very quickly: Phantom Lancer, Chaos Knight, Naga Siren, Meepo.
- Heroes that rely on illusions typically spawn very strong illusions inside your Wall of Replica that you should use against them or their team.
- Melee heroes have a hard time laning against Dark Seer because of Ion Shell
Works well with...Bounty Hunter
- Ion Shell on Bounty Hunter allows Bounty Hunter to do a lot of damage while staying invisible.
- Elder Titan gets along with Dark Seer for the following reason - Echo Stomp synergizes well with Vacuum and both heroes' ultis.
- Ion Shell on Riki allows him to do a lot of damage whilst staying invisible.
- Dark Seer's Ion Shell along with Moon Glaive gives both Luna and Dark Seek big advantage to farm, while Surge gives Luna bonus movement speed to escape pursuits or to chase fleeing heroes, Vacuum is also a good setup for Eclipse.
- Since Sven's Storm Hammer can stun enemies in a small area, Dark Seer can use his Vacuum to pull enemies into the spot allowing Storm Hammer to hit all enemies at the very spot.
- Sven can kill heroes that got pulled together by Vacuum very quickly due to his Great Cleave
- Surge is a partial solution to Sven's early game lack of mobility allowing him to close in on his targets with ease.