To beat each level, you have to kill all enemies to end the level and move on. You only have limited ammo in the form of gas tanks, so have to make each shot count. It is advisable to scout the level first and collect the weapon upgrade, as these will make the enemies go down much quicker and easier. Beware of the lava pits as they are instadeath, and keep an eye on your jetpack fuel to make sure you aren't caught in a pinch.