I have just recently started playing this game again and was wondering if anyone had any advise on how to effectively use this group. My main problem seems to be the Tau early game due to invisible enemies. I can survive standard difficulty about 70/80% of the time, but others I just get beat down so quick. --- The hardest part of a zombie apocalypse would be pretending it wasn't awesome...
I'm not generally an Eldar player, but I've played enough of the game to give some tips. And, as you're having problems with Tau and I'm a Tau player, I can offer some advice there.
-If you're having a problem with stealth, Eldar have (IMO) the easiest detection methods. Add a warlock to your basic Guardian squad and you shouldn't have any more problem with those suits.
-Upgrades are good. Eldar have a lot of upgrades, probably more than any other race I can think of. Make good use of them, and make sure you're getting what will work best.
-Make use of specialized units. Eldar are highly specialized, which is both their blessing and their curse. Guardians can keep the map held down for a little while, but as the enemy begins upgrading, you'll quickly find that these fragile squads won't cut it. Dark Reapers and Howling Banshees are both very good at what they do, so use them, and use them well. However, make sure they stay in their element. Dark Reapers in melee combat may as well be dead already.
-Be aware of the infantry's frailty. Have you looked at your troops? Eldar are frail creatures, and generally, getting in a firefight with an enemy like Space Marines or, worse, Necrons, will result in serious casualties. Eldar are meant for hit and run tactics, so use their speed and don't try to entrench unless you have the firepower to do so.
-Use vehicles. I can't offer too much advice here, but, as a rule, Fire Prisms are fairly effective. Wraithlords, as walkers, also do a decent job.
-Attach your commander to your Seer Squad and wreak havok. It becomes a very powerful squad together like this.
-Use the webway. This is a huge advantage for the Eldar, so use it! Stealth bases, heal units, and sneak units close behind an enemy base. It doesn't matter how you use it. Just use it.
-Play to the opponent's weakness. Using your specialized squads to do this will give you the advantage. For example, against the melee heavy Orks, try keeping them off of you using Dark Reapers (Banshees can dish out damage, but won't be able to keep up with Ork health), while against Tau, you should exploit their weak melee ability and send Banshees in after them.
Hope this helps. --- Life is but a dream. Death is an awakening.