I just skipped the whole 4th floor and just dug down. Luckily found a room with skeleton / wither skeleton skulls, got 5. I don't think I have the armor yet but maybe make some regen and strength potions. Or maybe I could make tons of octuple compressed cobblestone, enclose the wither in it, and hit it from an opening.
Hrm... first thought "Hey I should mention that the darksteel-stuff from ender-io is witherproof and easier zu make" ... then I found out you can use the compressing drawers for super-fast crafting the compressed cobblestones...
@NoxVindicta:
that's why I always close my smeltery on the top somehow... that never anything accidently runs in ^^"
@RHSean:
Well you can use steve's carts for making one (cart'hull + drill + storage, then you need a good way for making a an automated cobblestone with lava+water that it can mine it)
Or when you'Re lucky enough to get a diamond-pickaxe from monsters (you need to be really lucky if you not have some kind of monster trap for killing masses of them) you can use the block-breaker.