Note that most of what I say may be less relevant once the 6th Editions rules for 40k are made, but here is some general advice when facing necrons and building lists.
-Having long ranged units that can damage infantry. Most of their guns are 24", which means that scatter lasers, EML's, prism cannons, and doomweavers are all useful tools to dispatch their infantry.
-Assaulting their vehicles is the easiest way to destroy them, but their infantry can tarpit our squads somewhat easily because of their low saves, resurrection protocols and their high strength, toughness, and weapon skill (generally speaking).
-Jetbikes with farseers/warlocks can assault and destroy most necron vehicles easily, though its most safe to assault annihilation barges/doomsday arcs as these vehicles are not transports.
Basically I suggest that you have some long ranged shooting and at least one unit that can hold its own in close combat. Generally this would be a large unit of scorpions, harlequins, warlocks, or banshees with one supporting character (farseer is preferable, you could add in Yriel or a pheonix lord if you want to really invest in this unit).