Well, if he's using the portal to get the second WBB roll, then he can't use the particle whip on you eh? and if you're in close combat he can't use it on you then either.
He can only teleport one squad at a time as well and pretty much any Space Marine Squad should be able to down 1/3 of a necron squad in one round of assault.
Unless you roll crap
He also can't teleport them until his movement phase which means that you should be able to get at least one good round of beating necrons in assault and if he doesn't have a res orb around, power weapons kill out right and he can't WBB.
Just some things to think about.