Sorry to break it to ya, but if i remember correctly, he has 3 forms!
1st one was pretty simple. 2nd one was pretty hard.... (you gotta save your energisers!!!) and the 3rd one is just a b**ch, cause you can only damage him with the largest charge your buster cannon can fire....yes, the one after you get the upgrade. And each largest charge buster cannon will only take one bar off his life. You have to shoot him like.....uhh...30-40 times before he bites the dust.
Enjoy, mate.