item you will need: time in a bottle with at least 10 mins, a strong pickaxe,recommended a shovel, strong gear, weapons and you will need to have a way to walk through walls my method was phase step from wizardry bob.
so to get to the boss you will need to find a spot you can dig outside of the fortress and dig down until your near/at the bottom of the dungeon that's below. once there find a spot where you can phase step into it, and start the boss fight like normal [see a slugentower guide to find out how] then once that's done and the cage thing has fallen use your time in a bottle on where it fell and the boss should be and the boss should pop up. Happy killings.