I am just beginning to put a larger Necron force together. You seem to forget that Necrons were designed so that having a unified force to make it stronger. When a Necron player has learnt how to master their force they (apparently. Read WD when the necrons came out) can be come nigh unstopable. The fact the Necron Lord can take Veil Of Darkness, Ressurection Orb. He can start to move whole units from one side of the board to the other. Place a Monolith down out comes a 10 man Immortal squad (20shots Str5 AP4), f#*k. With the units that don't benefit from WBB, use them as the punching bag of the enemy. Say you have 2 Tomb Spiders instead of them hiding at the back of your army use them as spearheads for your force, T6 W2 add a particle projector for free, with 2 damn, they are very anoying to normal troops. Flayed ones, can make a mockery of a SM squad with ease, and their guys don't get back up. Also when using Necron warriors keep them out of reach of Battlecannons and AP3 weapons, power weapons etc. Always take 12+ Size squads, and use cover to attack back. When facing a Close Combat army stay just out of reach and hammer them with Gauss rounds, for Shooty armies, close in as quick as possible, and assualt.
Remember every army has it's weakness and strengths.