AOE or insane DPS, pretty much. Don't let the side-blobs get out of control, but don't ignore the main mass either. AOE makes that really easy, but if you have some passives or items that give you bonus attacks/turns or disgusting damage output (like 20+ damage per basic attack) then you can manage without it. Bringing a ranged build helps loads. If you're desperate, spam familiars and distract the blobs with those. (Pics rel.)
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In general, do
not try your luck with a full-physical team if you're attempting Act 3. It's maybe a bit manageable
in the past, if not ideal, but
the Future will fuck you up something
fierce. I do think the latter might be better for thieves, though, since it relies loads on Shield to bloat health bars. Debuffs are also good, even despite
a lot of Das Future shutting them down completely, and I've steamrolled entire sections just with that one debuff mutation + Necromancer + Spread Sorrow. It is probably one of my favorite tactics in the game.