I found him a breeze at level 49. And with bubble belts.

But even at lower levels I have been having a very easy time with bosses by using "Drain". It heals your guys AND depletes the fiend even if it is a flier or darn fast like the Vorpal Bunny.

3 characters doing Drain will pretty much suck out all the HP of most bosses even the ones with Regen and Cura / Esuna.