1st boss dead space iphone?

Ok so ive just payed like $10 or something stupid for dead space on iphone, and it is a good game except for when you get to the first boss, its right after a body falls from a girder and this big brute thingo jumps down. I have bought max ammo for all guns, and then go in, unload every thing into him, then throw all the explosives, knife him a few times, and still he doesn't seem to have a scratch. Could someone please help.