You only get a double WBB if you dont die... 27 bolt gun, 54 lasgun or 24 fleshborer shots should kill them (hell my dakka tyrant should eaily kill them in 1 turn if you come within 18" + 6" move). Ok yes I didnt bother including the lord in this so you'll actually need more shots but you get the point- a relatively small amount of short ranged firepower will kill these guys easily.You mean like your overrating a T3 swarm with a 5+ save and no Necron special rule?
You must be damn unlucky to lose 3 Units of Wraiths a turn, thats 9 3+ Inv saves with a double WBB.
Multiple units is harder to get rid of but still not hard (and if you portal them they'll normally be too far away to charge.. the monolith cant keep up with them).
Scarabs are a hell of a lot harder to kill- in the open they are 3W instead of a 3+inv and 1 less T (but still with 5+ save against some shooting) but where they shine is if they turbo boost- automatic 2+ cover save... unless the enemy brings flamers or gets into combat the unit is almost unkillable.