Tactix skaven vs ONG

My regular sparring partner has a cheesy little trick. There might be more to it but from memory he likes to bring a big horde of night goblin archers with a spider banner bsb. Only a small difference with the poison affect but the horde means the law of averages really catches up with me. I can never deal with the tar pit because of screening units employed against me that demand a response like mangler squigs and the like. This means they get several vollys off and often remain out of reach the whole game, decimating my units.

Im very much a rookie and he is a few levels above me in terms of grasp and tactics.

I would like to appeal to the community for ideas to deal with this.

Im not quite sure how crush embedded BSBs that are out of reach but since he loves this tactic and will most certainly do it again next game.

I imagine the go is to bring an assassin and try my luck?

Does anyone have an idea to help me counter this??

If something i have said doesnt make sense then i am probably wrong fyi. I have only ever played like 4 or so games and really struggle with the flow Ta