At the 'Bonus Content' menu select the 'Enter Code' option and input one of the following codes at the 'Secret Cheat' screen. When the code has been entered it must then be activated at the 'Secrets' menu.
Unlock ALL Combos: Enter the code Swampfire, Gwen, Kevin, Ben.
Invincibility: Enter the code Kevin, Big Chill, Swampfire, Kevin.
Infinite Aliens: Enter the code Ben, Swampfire, Gwen, Big Chill.
Level Lord: Enter the code Gwen, Kevin, Big Chill, Gwen.
Meteor Blast Move: Enter the code Big Chill, Swampfire, Ben, Kevin.