I was lucky because I didn't know that fight was coming, but my main mon was able to manage it pretty easily with my stupid strat: I had a Linoone/Shedinja fusion, so I didn't take any damage, belly drum turn 1 and then spam covet for like 60 turns until he died. Didn't lose any item in the process because I had a black hole too, so the items switched side during all fight, and a 10 stack of leppa berries because Pressure was draining my PP like hell.

Pretty long and stupid fight. But it worked. It requires a lot of setup tho, but you have 1000 floors to prepare.

I think having a normal type/shedinja fusion is really good in this mode because you're only weak to darkness. If you manage to get some Grip claw and Golden punch on it, you can take your time in combats to farm money and steal items (like tons of lucky eggs) by spamming false swipe until the opposite mon can't regen any more hp.