Gray Knights VS Eldar

hi the main person i fight is a friend of mine who plays a fast eldar army with lots of bikes and almost everything with transports (i am almost certain he cheats point wise) and i find myself always to slow or to small to beat him. can any one help me? my biggest problem is that he takes a farseer (whitch i cant kill because of the dire avenger squad, Avatar and half his army protect any ideas?) that has runes of warding which stuff my pskic powers

Also he is one of those pains who read between the lines of rules and bends them.

ps. we always play games of 2000pts and always annihilation or Objectives

First Question, do you own an eldar codex, if not get one. (download one there should be a new one coming out in the new year (No I do not nkow when) and spending money on something that will be useless in 4 to 6 months is never a good idea)
Second Question, what is in your army now.
Tips on eldar, aviod a magic based army, shoot them and use your own avoidance tactics, rember in the new rules he cannot charge out of his transports, eldar do not have big ranges on their basic troops so take out anything with good range first (over 24"), then work on the bikes from turn 2 onwards, you do not have to wipe out your opp just beat him.

Interceptors with incinerators will toast his aspect warriors en masse. They're also pretty handy for chasing down jetbikes because they can move to get LoS after they do their 6" fall-back.

Remember your psy-strike missiles on the ravens, though you can only launch two a turn now they're designed for exploding psyker heads.

The avatar is a daemon so remember that the majority of your army re-rolls 1s to hit and wound against it.

Your army has a 5+ deny the witch along with the aegis rule. This will help stop 'doom' and 'mind-war'. If he's using these a lot keep a dreadnought up with your main body of troops to use the reinforced aegis.

Eldar transports are 100 points for the most basic version, so if he has more than 5 bombing around he's probably bending the rules.
Consider taking a grand master. The strategies can't be stopped and allow you to get a unit to (hopefully) flank the farseer's unit and shoot him full of psybolts as one of the closer targets. Or send a stormraven. You can be the most powerful alien witch in the galaxy but a multi-melta blast to the face is still going to hurt.