So there are a couple of skaven players around at my local store who I don't think have ever really grasped how to use skaven properly. They never really win and just seem to get demolished. They don't use any slaves which I suppose could be part of the problem.

My question is, what makes skaven so OP? I mean sure they can hold you up in combat for ages with their slaves but what do they really have apart from hellpits to do the damage? All I see the slaves are as holding the units so where does the real damage come from?