Ah, Necrons. Annoying, but very much beatable. There are a few simple things to look out for.
-Does he take a monolith? It's a cool model, but it's a liability. That's 235 points' worth of Necrons he doesn't have on the table, so you're 235 points' worth closer to phasing him out, figuratively speaking (since the value isn't what matters, but the numbers...)
-Are there destroyers? Again, excellent models, and vital in the army. However, they're awfully hard to screen, and the sooner you shut down those three-shot guns, the better. Besides, their speed tends to take them out of range of a lord's ressurection orb. If they bother spending points to put a lord on a destroyer body purely to run around and babysit them, then it's the same thing as a monolith, really... that's that many fewer Necrons to worry about.
-If my last two points weren't any indication, focus on units with the Necron special rule. That means that while Pariahs and Tomb Spyders may rip you a new one in close combat, they might as well have never shown up when you whittle down the army to the phase out number. Focus on Necron Warriors-- they're the mainstay of the army anyway, and are quite dangerous at all ranges thanks to a Space Marine's profile with a more tactically flexible weapon.
-Outshoot them. Yes, Necrons are thought of as a shooty army, but ultimately, they're only really dangerous at 24'' or closer. "We'll Be Back" is most dangerous when you're playing a game of attrition against the Necron army in close combat. With a Space Marine's profile sans the Initiative, the basic Necron Warrior is not quite what I'd call helpless in base contact. If the Necron player knows what he's doing, he's going to march forward in a phalanx formation, firing when able, and he's going to bank that you charge him. Yes, you'll knock a few warriors down, but a Lord with a resurrection orb is going to ensure that even a power fist's hits are recoverable, and you're going to find yourself slowly whittled down while the Necrons continue to stand back up. Specialist close combat units like Flayed Ones aren't fighting back at range, either, so if you see them on the table, they're a good thing to aim for.
-In any fantasy/sci-fi/horror thing I can think of, killing the source of the undead puts a stop to the whole ordeal. Necrons are no different. While they may not crumble when you off the Lord(s) who's toting the resurrection orb, it does make your more powerful weapons a hell of a lot more useful. In fact, you might not need to rely quite so heavily on bolters or lasguns winning the day if you can off the lord if you've got a few plasma guns, a power fist or two, or even single shot, low AP weapons sitting around. While the Lord(s) are still up, you'll find you're going to put down (and have them subsequently stay down, at any rate...) more models with the grunt weapons than you will those single shot wonders like lascannons.