When corrosive is shooting missles at you there will be a yellow box with 1 stanstard energy pack in it.Try and get corrosive to shoot the box 2 times and you will also find a wrench in some of them (hint:look on the right of the coloseum!).After that go into one of the small huts in the center of that area and dismantle youreself then that idiot (corrosive!)will think you're dead when he blows up the hut then go to the last level (BUT...that's part 1!)
When you are using the slingshot, the best way to aim is with your antennae (spelling) When you pull back your slingshot, line up your target with the tip of the antennae and let go. Your ammo should fly and hit your target, but be warned, it doesn't work at long range.