I thought I'd start a threat with examples of weapons that I'm finding are quite effective against my Necrons. more to be added later.
When using a missile launcher, try krak missiles. Necrons have a very nasty habit of ignoring low ap templates. Now, a demolisher will wipe them out effectively, but a frag missile will not. It's far better to fire a krak missile, which won't scatter in the first place, and punch out a necron model than to pop them with frag and watch them all survive the template.
Not to mention that if there's no necron lord nearby with his pissy little res orb, then the krak missile vaporizes the model in question.
So when fighting Necrons, either get busy in close combat, or get kraken.