After receiving all the above achievements for the DLC, you will descend one more floor and fight a very tough enemy. There is no save point down there, so if you fail you will need to run all the way back down (just turn-tail from every battle).
Now, this guide will explain how to do this battle in a mere 3-4 turns. That may seem insane for such a tough boss, but it is by far the best way to deal with him. He heals for 20k+ after every single turn, so you need to deal heavy damage quickly or he will outlast you, as he also has very big attacks on top of his auto-healing.
One thing I might suggest that the guide linked above does not, is to sacrifice one double-cast on each of your first two turns to put Reflect on all of your immortals. This will reflect away his Annihilation spell. You might also want the skill that absorbs physical damage on everyone, as this is what his other super-move deals out. Finally, you may want to try spending the turn after he casts Reflect using small spells like Flare to eat away his Reflect spell (it can only reflect 3 spells), this way you won't have to get lucky with your immortals triggering a 0-damage.
When I succeeded in killing him, I got extremely lucky (and almost everyone I've seen do this has as well) and he used a Reflect spell when he already had Reflect on himself, so this not only wasted his turn, it also triggered his third reflection. On this attempt I was using all Reversas instead of trying to eat it away with Flares, so since he broke it himself, I got in a few extra Reversas and killed him off. The battle really is based on putting as much in your favor as possible, then just hoping you get lucky.