Stalker won't hold them up much.
Worst case scenario:
20 stealers on the charge-
60 x 2/3 x 1/6 x 1/3 = 2.2 glancing hits or ~ 1/3 destroyed stalkers, aka probably surviving the worst case scenario.
But if you let 20 stealers charge, you are an idiot so thin them out a bit before the combat to improve your odds (which are fairly good even in the worst case).
Stealers are a potent CC unit, but nothing special as far as Necrons are concerned. If you are I2 it doesn't really matter if your opponent is 2 or 4 points better than you.
I'm not really sure what this means- a large stealer squad will butcher any infantry in the cron army. Cover and FNP seriously diminishes return casualties and they can infiltrate for a turn two charge. This isn't true of gants or the like.
Wraiths are possibly the best counter-CC unit the Necrons have, C'tan and Lychguard possibly the second and third but I have not decided in which order.
All of which will fold like chocolate against stealers, except possibly for wraiths if you can tie up most of the squad with the walker.
Quantity over quality is what is needed for the Tyranids to beat Necrons.
A meaningless platitude. Woop.
A bit less posting of mantra and a bit more thought (and experience?) is required methinks, especially since large stealer squads are something I expect to see in competative nid armies. They were definately in mine, and the thing they hated most were av13 walkers.