Go to the Main Menu (with the Thing's mug showing) and go to Options Menu. There, highlight the
training bar and press all 4 trigger buttons at once (L1+L2+R1+R2). This will
allow you to access the following items on the options screen:
Invincibility, Level Skip, Unlimited Replay, and Big Boy (which lets you
play a giant character or a tiny one)..
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Thanks to Revolution readers Kauza, Thor660, Humphrey Ongkos, Jojo, and PPol!