As a former eldar player, i can tell you that the battleforce is great.
Next thing you should buy is a farseer definite, GET FARSEER AHAHAH!!
Some tactics could be: to put the farseer with the dire avengers in the vehicle you get with the battleforce and cast guide on the dire avengers ( lets them re roll failed to hit rolls) and to also cast doom on the unit you are going to shoot at ( lets you re roll failed to wound ), you then get the dire avengers out and use bladestorm which lets you fire 3 shots, 3 shots times the 5 avenger = 15 shots with at least about 12 hitting, these shots should negate most things armour saves especcialy swarm armies.
After you buy your farseer get another squad of dire avengers so they can also follow up behing the vehicle and bladestorm the opponents, but these guys cant re roll failed to hit rolls because you farseer would of casted it on the unit inside the vehicle.
Once you shoot, get back in, go somewhere else and keep doing that.
Hope that helps