Since XIII is running off a varient of the Unreal engine, you simply need to add a keymap to the GOD cheat code.

WARNING: This requires editing .ini files. Be sure to backup any files you edit.

Go to your XIII\System folder (i.e. C:\Program Files\Ubi Soft\XIII\System for the standard install.) Open up user.ini in notepad.

Find:

Comma=

and replace it with

Comma=GOD

Save the file and then run the game. Pressing the Comma (,) key will turn on god mode. This should work on both the demo version and the retail version.