Sink the Shark Submarine
You must sink the Shark Submarine at the end of level 2. First, kill all the tan soldiers that appear. Hold L1 and press the Right Analog-stick to aim for the gun turrets on top of the submarine. Destroy the turrets to sink it.

Staying alive:
Never stay still, even if you are trying to shoot someone or something. If you move around, it will be harder for the Tan army to hit you; same with the Bosses. If you miss an enemy, turn around and come back for another pass, but do not stay still. For heavy populated areas, go forward but weave left and right as you do so. This will help keep you alive until the next health bag.

Good weapons:
Try to get the Bazooka and the Flamethrower's power to maximum. Use the Bazooka as your main weapon. It can blow up obstacles as well as cause heavy damage. If you find yourself unable to kill an enemy near you, you can quickly change to the Flamethrower and melt them, then change back to the Bazooka.