When you reach the strange glowing door (how DOES the boss fit through there?), you need to warp back to the overworld and buy the Legendary Equipment.
BOSS: Fear Incarnate
This is it, folks: the monstrosity that has besieged Monster World in every single game. Finally, you meet him face-to-face, with no underlings, fancy space technology, or conveyor-belt buzzsaws between him and your sword blows.
But first, a flock of four evil pets attack you. Bash them a few times to turn them back into happy, lovable Pepelogoos. Awww....
Fear Incarnate then talks to you.
You must hit the two floating eye stalks. Choose either corner and jump-attack that eye.
Fear Incarnate can use a petrifying attack, which lets him attack your helpless chilselled visage. Note that you stay rock-solid (enough with the puns already!) indefinitely until he hits you. I don[[[']]]t know what happens if you are in an unreachable area and he freezes you. If you really can[[[']]]t continue, I suppose you have to restart (you can't use the funny Genie during a boss fight!)
When things look grim, Pepe flies in. He stands in front of the eye, so the petrifying attack is blocked. As his last use of the eye attack, the monster freezes poor Pepe!
Continue attacking as before now his petrifying attack is disabled. Eventually you'll defeat him.
(TODO: Pepe funerary scene)
When it's all over the genie thanks you. And he tells you to go outside and play. According to him, this is the end of the series. And, currently, it is.