Well, a good thing for use against nids, GK and IG are Fire Prisms. Take a couple of them and they can splat swarms and take wounds of MC's, whole IG sqauds (especially heavy weapon platoons) and take out their light vehicles, and of course combine beams to obliterate SM sqauds.
Dire Avengers with an Exarch with 2 Shruiken Catapults and Bladestorm are good. They'll be able to lay down lots of fire on the move, and on the final shooting phase before they are assaulted or shot back at they can bladestorm to take down a big swarm or a sqaud of Marines.
Against tanks Swooping Hawks are very nice, give 'em haywire grenades and they can easily take down 2+ tanks a game.
Eldar tatics are: MOVE! Unless you are using things like Dark Reapers or Rangers, you want to constantly move your troops about to exploit your assault weapons bonus. Tool Fire Prisms up right and they can move more than 6 inches (and so claim the 'downgrade to glancing hit' bonus), with no negatives from doing it. The only times you should stay still are: if you are in cover, and need to stay there to avoid being blown to bits by medium AP weapons, or a swarm is running towards you! Stay back, and then just move backwards firing at the enemy.
Well, I'll probably be able to think of some more things to help later... I hope this was helpful!
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