Necrons are already easy to phase out. Whether you take "Necrons" or not, if your opponent has looked at any sort of tactica for fighting Necrons, best bet you will loose.
Now, I took Necrons to BoLScon '09 and placed in the top half (One place above Droids_Rule) and I used 4 Pariahs, a Monolith, Deceiver, 33 Warriors, 6 Heavy Destroyers, and 10 immortals. I used the Heavy Destroyers to pop things like Defilers and Vindicators before they wrecked too much havoc, and rapidfired most thing to death. It worked a lot better than I fought. I managed to keep my Warriors in cover, and basically did the opposite of Phalanxing. I sent my Deceiver and Pariahs at the enemy, and stayed back in my deployment zone, and made them come to me. The Destroyers demolished all light armor, and Mech lists ended up having to footslog. You'd be surprised how many people will charge into cover without frags, and realize that Necrons are essentially really slow marines, and ended up loosing combat. If you take your army, and plant it somewhere, you can hold that ground for a LONG time.