Wario Land 4: Part 2: Boss 1: Spoiled Rotten
After beating all of a Passage's levels and collecting all four Jewel Pieces, you can access the Boss Room past the Mini-Game Shop.
That Black Cat will follow Wario in, and before the actual boss room, itself, you go one more shop you can go into: The Item Shop.
This has a Shopkeeper that looks like a Mr. Game & Watch with eyes and fingers. You can use the Medals you won from your Mini-Games in order to buy special weapons that will instantly and automatically get used on the bosses to slightly damage them before Wario can have his way with them, himself.
Or... you can pay no Medals just for free smiles and sounds. Seems like the Shopkeeper really likes Wario!
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The first boss of this game is a giant moving eggplant called Spoiled Rotten. He's a simple sleepy creature of a veggie that can't even really fight you on his own without using minions.
You can attack him from either the front or from behind... until he gets hurt enough and suddenly freaks out into a real monster! Even then, he's still not a hassle. You just can't hurt him from the front, anymore. So just hit him from the back a few more times and then you win.
Did I mention that all boss fights are under a time limit? There's a reason for that that this boss doesn't have, but all of the others will.
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After beating Spoiled Rotten, and this clearing all of the Entry Passage, Wario will now gain access to four more Passages, each of them with a bigger set of levels to complete before you can reach their bosses.
Besides this, this game is really quirky. Even more than any of the previous Wario Lands were. I can mainly see this in its music and sound effects, as well as the even weirder characters Wario will come across, this time, including that particular Shopkeeper.