Witch Hunters aren't an anti-Eldrad army. They are an anti-psyker army. They also have access to assassins. Eldar have more access to psyker units than most other armies. Like I said, take WH allies. All you need to do is give up 1 HQ or elite slot to get the inquisitor and his retinue and you should be able to take out Eldrad's unit no problem. You could be playing any SM or IG army with WH allies. Black Templars would fit the idea the best though.
*sigh* I have to point out that despite what you see in games, lasrifles win wars. Just because lasrifles suck at killing superhuman killing machines in power armor does not mean that they are bad weapons. You are shooting them at the wrong targets. The most common enemy the Imperium faces is lightly armored humans armed with lasguns or autoguns, NOT SPACE MARINES.