A good strategy is to keep switching between your assault rifleman, rocketeer and sniper as the game limits your three characters energy.
Weapons and experience
Always upgrade your weaponry between rounds whenever you can. You can get extra Experience by replaying completed games.
Keep an eye out for mines as well as avoiding the explosions and the shots the game automatically fires at you. If you come across a mine just jump next to it and your character will shoot it out of the way.