Primordial Soup (Bonus Stage)

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.

Story
Upon entering the stage, Laharl and Etna find a book lying on the ground. Laharl doesn't want it, suggesting they burn it. But Etna tries to convince him otherwise, spinning a tale of a sacred tome from another netherworld that can make any wish come true. Laharl sees right through her attempt to use him as an experiment. So Etna decides to just try it out for herself; However, as soon as she touches the book, they are confronted with a very angry Zetta.

Strategy
This stage has a lot of enemies and geo symbols. Luckily, the geo symbols are only level 150. And if you're attempting this bonus stage, you should be able to one-hit them by now. Also, the stage is divided by a 'no entry' area, so you don't have to fight all the enemies at once.

Because the base panel is located in a closed off area with 'Ally Damage 80%' in effect, eliminating these geo symbols is the first priority. However, be aware that if you do this, the whole area will turn green, and thus the 'Enemy Boost x3' geo symbol will be in effect. If you have a decent developed magic user, you can use magic to get rid of this new geo effect. Ideally, you're able to get rid of both geo symbol effects in the first turn.

After that, you can start elimating the enemies on your side of the map. Beware there is a small chance enemies can 'warp' over to your side of the map, if they should end their turn on a yellow panel. For example, Zetta probably will. If you don't want this to happen, you can either occupy the yellow panels on your side of the map before ending your turn (and thus preventing any incoming warp), or you can eliminate that geo symbol effect as well.

Once your side of the map is clear, you can clear out the rest. Magic users are very handy for this part, as they can safely target the enemies from a very long range. If you, for whatever reason, would prefer melee for this part, you can throw a character to the other side. Just make sure to throw the character over the water, as the 'no entry' symbol also prevents entry by throwing characters.

Zetta has a rank 40 (legendary) Nemesis equipped, and also a legendary Arcadia. Stealing his equipment will weaken him quite a bit, and the items are also very nice to have. So feel free to rob him if you have a decent leveled thief.

Disgaea DS
Defeating Zetta will result in recruiting him as a playable character. However, Zetta will always be level 3200 when he joins, even if he was level 9999 when you fought him.