When I was eleven years old, it was a very good year, and I can remember my daily routine vividly. These were the years before I owned a Sony Playstation, and I used to venture to my friends house - everyday after school - to watch him play through Final...
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.