You've got about 2 turns worth of shooting (3 if you're clever with your squads), before you need to worry much about being charged.
Just shoot the kans with everything that *can* hurt them, and whichever boyz squad the meks attached to at the time with whatever cant hurt them. remember necron gauss rule of auto-glancing on a 6 to hit... They can really bring the hurt to the Kans, since they only really need to be immobilized before you move on to the next. A stun is as good, basically anything that stops them moving. At some point, he'll need to move the boyz out in front of the wall, just focus everything on one boyz squad to bring them down. (Stick to the rule of kill a squad, then move on)
With my tyranids, i more or less employed this tactic - though i wound up killing the boyz squads off with biovores, then watching whatever was left assault me and more or less get slaughtered. Key to survival is killing the kans though. Key to winning the game (assuming objectives) is killing the boyz without losing much.