This is the ultimate shooting game. Stand still and shoot everything that comes towards you. There are so many aliens and monsters to kill, and you do, as soon as they are within reach. Fortunately you have enough ammo to last you. You do have to change guns every now and then, you don't want it to block because it overheats do you? Don't forget to shoot the bombs on balloons, because the balloons will snap right above your head. See how far you can get.
Shoot with the mouse button, aim with the mouse, choose your weapons with the mouse. Use the W to stand, the S to crouch.