well, in terms of tournies, i'd say DE armies, tau armies, and eldar armies could handle them with no problems, since all three are pretty shooty.
Tau just have pure firepower and should phase out necrons before they even come close enough to hurt them.
Dark Eldar have speed and one of the deadliest HQ squads out of all the armies.
Eldar have dark reapers. Need I say more? =P If i do, then howling banshees.
For the other armies, i'd say IG shouldn't have a problem. A tank or three will take care of anything.
SM and Chaos should be fine, it would just be like fighting themselves, except their necron counterparts would get back up.
Right now, necrons are slow. When the monolith comes out, that could be a problem, since that will give alot of speed to necrons. In theory anyway.