All you have to do is after shooting down the crystals, empty every clip of every gun you have except the bazooka at the monsters head, and then when you emptied all your clip, if the thing is still alive use you bazooka and fire it right at the monsters head. Remember hide behind the sharp pointy walls. Don't forget to bounce up on the bouncy thing to the top part of the wall. It is much easier. I beat the game 4 times already.

Well some people get the common glitch when he just stays in the air and twirls around and round. I also got that. It is best just to start a new game and try to defeat him again. For killing him is concerd all you do is shoot him. He has alot of life and will take a while to kill him, but hide behind one of the Pointy things that stick outa the ground. Try to get to one closer to the wall.
Also FYI: You haft to shoot the Yellow'ish Orange crystals on the wall befor you can actualy Kill him

after hitting him, the glowing orbs will decrease in number (the ones he shoot at you until theres only 1 weak orb per shot. then hit him a few more times and his head will bust open like a melon. after it opens use the alien jump platform to get above him and bazooka him in the brain when you see it.