An insanely easy way to beat this guy is to wait until he starts spitting toxic sludge at you, he'll glow a dark purple, and just dodge-roll to the left and right, then roll towards him as he staggers after the attack. Doing this can get you 3 or 4 hits.
You can also throw some of firebomb-esque things at him, which will slowly drain his health as well. You can also throw your spears while aiming at his gut, 2 usually stagger him, and you may be able to roll up to him for a few quick hits.
Also, make sure you focus either on his gut or one of his legs in order to get one of two achievements.