Unlock Hard AI
In modes Vs. Computer and Flash, after you have selected your character you will have to select your opponent. While your desired opposing character is highlight you can press and hold the L button and then press the A button. You'll notice that one more star is added to your opponents handicap. The words "Com Lv Hard" will appear in the upper right-hand corner of your screen.

Unlock S-Hard AI
In modes Vs. Computer and Flash, after you have selected your character you will have to select your opponent. While your desired opposing character is highlight you can press and hold the L button and then press the B button. You'll notice that two more stars are added to your opponents handicap. Also in the upper right-hand corner of the screen it'll say "Com LV S-Hard".

Increase Your Virus Population
In Marathon Mode sometimes you may need to increase you Virus Population on your screen. You can do this by pressing either the L or the R buttons during the game. This will allow you to continue scoring Virus point while still maintaining a somewhat clean screen.