This one was tough. I can tell you what I did which might be dependent on some of the skills you've bought, and I don't know if this is how you're "supposed" to do it, or if there's a way to weaken them somewhere else in the dungeon or what, but anyway...
I defended the first turn, then used the 100 TP skill that grants double turns on turn two, which stuns them all that round. Turn three, I used a 1500 HP heal and used the 50 TP iron knuckles skill to kill one of them to get them down to 3. 3 weren't enough to kill me, so I repeated healing and using that skill until all 4 were dead.
The two-turn skill seems almost necessary for this fight (unless there's a gimmick I didn't discover). It seemed like the 40 TP iron knuckles skill took about half of their HP as well, so maybe once you've got that buff up, you can just heal and defend in the same round until you have the TP to use that skill twice in one turn to end it.
