Before Microsoft and Sony do something regarding their future in the video game business, I wanted to write, and I've wanted it for a long time now, but other things kept getting in my way, and fearing that tomorrow might be too late, today will have to do.
Shoot everything at least once. That's how you earn the big bucks.
Clearing waves gives you bonuses, so try and kill everyone within the time limit.
Shoot barrels when bad guys are near them for quick explosive kills.