After defeating the Almighty Demon King it is now time to face the final enemy: BioMeka. You can reach him by pressing up at the belltower in the kingdom of Sheila Purapril and going left. Enter the elevator and once you are up enter the pod. The pod will bring you to BioMeka's Spaceship. His room is blocked by some seals however. To open the seals you have to kill all the previous bosses (except for the Sphinx and the Almighty Demon King) once more. This is quite easy now that you have all your new equipment. All enemies in this area drop small hearts, in case you get damaged.
BioMeka is the final boss you have to face, and can be quite tricky to defeat. First, when you enter his room, do nothing. Just stand at the left and let him hit you. When you are almost dead, the Prince of Darkworld will appear and knock out BioMeka's laser gun, then restore your health before leaving you alone to continue the fight. Destroy his armor in 16 hits with the Legend sword, you can use your Thunder to destroy the lasers. The lasers will respawn periodically, once you've destroyed his armor, the floor will turn into a conveyor belt with a circular saw coming out of it. Using Power magic will help you kill him a lot faster.
After he is defeated the game is over. You get to see the credits and after that all the characters you have met in the game will say goodbye one last time.