Beezelbub

This is one disgusting prick. You have to wack away its rotten body parts and the more parts that fall off, the more giant flies (his name was Lord of the Flies in the Japanese version) come at you. They will aim for you, so the fight gets more challenging as you move along. Hence the name BEE-zelbub (it's also a reference to Beelzebub, a name given to the devil). Go on the side platforms and throw Knives and Axes. The dangling of the body parts makes it a hassle to use Holy Water. It's cheaper, safer and you have better odds at hitting him than with the cross. Soul Steal and Tetra Spirit (or lots of summon sprits) are useful here also, especially if you cast the spells directly under him. It won't be efficient to try to hit continuously with your sword, especially if you have a liking for short blades. He's in the Necromancy Laboratory (inverted Alchemy Laboratory).