It looks like you could put an arachnarok on 2 50x75's, so you could probably make a 100x75 base so you don't have the extra flank size. And then, you could just make a 3rd model that is on a regular 50x75 (maybe a smaller spider or a guy on foot swinging his dead spider around as his weapon) that you could use as the odd-man when 1 Skillcrusher is killed.

I imagine you'd be able to play something like that anywh