Primordial Soup (Bonus Stage) contains an optional fight against Zetta. This stage can be unlocked by passing the stronger enemies bill 20 times, and then speaking to the Dimension Guide. When you do, she will mention some new activity at Stellar Graveyard, and thus opening the bonus stage. Zetta will be level 9999 if you enter the stage, unless you undo the bills (by passing the weaker enemies bill 20 times) again, in which case he will be level 3200. Just make sure to have spoken to the dimension guide first, before undoing the bills.