Defeat Lucifer to successfully complete the game. Then you may explore any area in the game freely. Additionally, Lucifer can be found again inside the Cave of Oasis.
After you beat Lucifer, go back to Nova. There talk to Uncle Hata and he will join you. It turns out that he is Michael.
In the world with the Luciferium, go to the northwest corner on foot. Explore the mountainous region to find a hidden cave. In it is the Omega Armor and Omega Sword(for Uranus) plus the Alpha Mail and Alpha Sword(for Kishe).
Make Better Monsters You can upgrade just about every monsterit . The best way to do that is to use Lich. Yep, 12 HP level one Lich. You will need to catch an Owl and Vepal (There are other ways of making Lich, but I found this to be the easiest) outside (and inside) the Tower of Magan. Then, choose the monster you want to make better and combine it with Lich. If your level is high enough, you'll get a killer monster in return