Guard, Nids and Orks.
These guys are always going to give us grief, Eldar weaponry is fine for laying smack on a single unit, but when there are 8 squads of Guard infantry alone (not to mention the other supporting squads of special weapons, heavy weapons teams and the like) in standard games, we're looking to figure out either:
1. is the objective worth more than the effort
2. is split fire viable at the correct ratio
In the first part, I watched a battle recently where the Eldar player was hilariously outnumbered, and even by Turn 5, the Eldar Autarch, a Falcon and Wave Serpent were about the only units left. Eldar player 'won' 8VP to 5VP. On the battlefield, if the game had gone to Turn 7 or more, Guard would have wiped the Eldar off the board.
In the second part, if we run full squads of guardians, and split between shooting Guard Squad1 and Guard squad2, maybe we can break even in the war of attrition. In the current ruleset, a Commissar only forces a re-roll of Morale, so if a Guard squad loses 5 of the squad, it's a quick rationalisation to determine if losing the Guardsman to re-roll is worth it.
I'm the guy who at 2000pts has 8 Guard squads (one of them a 30 string Conscript unit), three Special Weapons teams and three Heavy Weapons Teams which I think is about 900pts. It is very hard to kill everything, and players who often run with 'Kill em all' as the game plan, simply run out of time.
The question isn't just 'How do we kill GEQ', but also 'How do we deal with swarms'.