The Warhammer Eldar army has a good selection of specialist troops, and fairly poor basic troops. With the Eldar army it is much more important to know as much about your opponents army list ahead of time than with other more general army’s. If you know your enemy before you make your army list, then you will be able to have their light specialist troops available. For example, if you know that you will be facing a tank heavy army, then you add more Fire Dragons to the list; if you are facing lots of power armour, then you add more Screaming Banishes that ignore armour saves and Dark Reapers with AP3 weapons. If you do not know the army that you will be facing, or you will have to face several different armies with the same list, then you will have to have several different specialist squads and use in game tactics to get the right squads facing the right target.
Warhammer 40k Eldar troops do not survive counter attacks well. When planning your attacks, plan to wipe out the opponent with the first strike. This will require you to split up the opponents force to to groups and focus on each group in turn. Use you speed to dart around the table picking off any vulnerable parts of his force.