In Ben 10 Ultimate Alien: Cosmic Destruction for the PS3, PSP, Xbox 360, and Wii, there is a cheat code you can enter that will unlock the Rath form to play with.
Note that once you turn on cheats, you won’t be able to get any achievements/trophies on that saved game again. You’ll have to either start a new game, or go back to a previous save.
To enter cheats, go to the main menu or pause the game and select the Cheats option from the menu, and enter PRUMUS as a cheat code to unlock the Rath form.