i just got this game the other day and i already finished it but im playing again its great.
To kill the boss, aim at the green bits of his body (head, chest and back) keep shooting until it turns from green to red. When he makes a shield around himself to regenerate health, destroy the spikes as quickly as possible to stop him regenerating.
Hint, use stomp (jump the press the punch button (A then B for xbox 360) to kill the underground creatures quickly.
I'm up to the last boss and I can't figure out how to get him to stay down. I have gotten him to the point where he puts up the shield, rases the spires, and calls a BUNCH of those underground dwellers. In fact I got him to this point 5 times in one round. But the dude kept coming after me.