Hi, I love shooters too, and to practice try the savestates feature. For example, if you have extra buttons on your gamepad map them to "System Load State" and "System Save State". Press save button after every successful enemy wave or every 10 seconds or so. when dead, press load
Also can map "UNDO" incase you make a mistake and save after death, then load again. On my Dualshock 3 controller I put loadstate to R1, save to L1, and undo to L2.
I like this method better than cheating, because you can practice and get very good at a game, and eventually not need to use them.
bye for now,
- dink