Actually, due to the nature of the guass rule, all necron weapons can hurt them, just not very effectively.
A tactic I developed while havn't a massive "how to beat necrons" brainstorm does involve grotesques. An archon attached to a squad of grotesques mounted on a raider zooms forward they all disembark, the archon deataches, and they both assault a warriors squad, the lord kills a few and wins the combat for the grotesques meaning the enemy automaticaly falls back because of the grotesque special rule, then your lord, being a seperate unit now, catches them with his I 7 because necrons are I 2. Whole squad dead, and they can't get back up from being killed in a sweeping advance, then incubi and a dracon smash into another squad, quickly get rid of the destroyers with dark lances and disintegrators as they pose a major threat, and without them, his army can't be very mobile and then you can run circles around him picking and choosing where you go, lances and disintegrators are priceless. Do NOT, I repeat, do NOT attempt to kill the monolith, it would be such a pathetic waste of firepower to try to even scratch it, much better to kill, a bunch of warriors and then he will phase out. I'd say do not use wyches as they rely on slowly beating down the enemy instead of just smashin them which is a bad thing against necrons because then the monolith can teleport them out of combat so anyone can then shoot oyur t3 6+ wyches and you'd have only killed about 2 necrons. Anything that can kill them quickly. Don't bother with any heavy destroyers as they would be a complete waste against us, only one shot, and against our vehicles they can still only cause a glancing hit. Keep your power weapon armed people away from wraiths, maybe get rid of them with a single wych squad if you really have to, since they can only have 3 in a unit they couldn't be that hard to get rid of, the best way to get rid of them would be models armed with poisoned blades or scissorhands so haemonculi would be a good idea, you wound them on 2's which means they will be taking a heck of alot of 3+ invunerables and with only 3 models that sucks.