Cheat mode
At the "Bonus Content" menu, select the "Enter Code" option. Enter one of the following codes at the "Secret Cheat" screen to unlock the corresponding cheat option. Then, enable the unlocked cheat at the "Secrets" menu.

Invincibility
Enter Kevin, Big Chill, Swampfire, Kevin as a code.

Infinite Aliens
Enter Ben, Swampfire, Gwen, Big Chill as a code.

All combos
Enter Swampfire, Gwen, Kevin, Ben as a code to unlock all combos.

Level Lord
Enter Gwen, Kevin, Big Chill, Gwen as a code.