Not sure if this is intended, so you'll have to inform me, but if you get slow up on a mob using Ramei, and then use hell thorn, and then Ramei again, you can double slow anything including bosses, and it trivializes any combat. I simply put slow up, poison, and slow again and just keep recasting slow while waiting for poison to tick down the HP of the boss. I have done this through 2 continues and has worked through the whole game each time. Also seems to be the only faster way to deal with creatures wih 4mil+ hp.