when I first started I had problems with the puddles also. My buddy who stopped boxing swears that you can almost kill him from when he first spawns to grows big. I have yet to make this work out but it might be something to try. If you watch during the fight he/she starts very small and is targetable at this point. The purple thing grows bigger quickly (~4seconds) then starts attacking/pooping purple puddles. The idea is to dps the hell out of it before it even attacks you.

Good luck it will just take time and you will get it.