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I always play against my friend who uses Eldar, and i always find that he smears the table with my Necrons. What gives me the most trouble is generally his farseer hanging out with his War Walkers, or his harliquins (with shadowseer) that i cant even shoot at. It's frustrating and I've only beaten him once.
Anyway, I'd like to see what kind of tactice other people have for taking down those long hated enemies of the Necrons.
Get really cheesey. Bring in 2 monoliths, a C'tan, and a bunch of destroyers.
Make him cry.
The next issue is whether or not the eldar player is using a fast army. If he is not, then look into a lord with the viel, and a monolith. speed up those warriors. lock stuff down that you know you can handle with them. Then support them with stuff like wraiths or whatnot. Look into dismantling the army a section at a time. Just remember, the Eldar play like a concert. If you can "Manage" to disrtupt their concert, then you are moving in the right direction. If not......then keep on trying.
I've had a problem with Eldar in the past but I've found a few things that helps to roll over them every game.
First. Is LOTS of Immortals, they can get way more shoots of at them and wounding them on a 2+ is priceless.
Second. Hug cover, Eldar can deny alot of armour saves so keep your guys behind cover.
Third. Is Monoliths tend too walk right over Eldar (Keep away from wraith guard or swooping hawks)
Fourth. This is the most important one, Eldar are a very specialized army and every unit has a very specific strength. The best way to counter this is to basicly lock in on a single part of his army; blow the freakin 'heck out of it and repeat each shooting phase you have is another nail in the coffin with this tactic.
Now my finale advice is to be careful with playing these tactics, I actually made a friend completely stop playing Eldar cause I beat him so many times ;? Like I felt bad how bad the punishment I could dish out to his army, I literally cringed whenever I had to shoot with my immortals. For once in my life I prayed to the dice gods for all 1's