Even though it takes longer for him to level-up because of the lack of enemies with many experience points and the fact he needs to move around more than the Mario Bros., Bowser is now at Level 50. Previously, he was at Level 43. I found out that I need to leave the Peach's Castle building so the enemies I defeated can re-spawn. Like Airway for the Mario Bros., I have to enter, leave, and re-enter Peach's Castle to fight the re-spawn enemies.

During my time leveling-up Bowser, I learned that I can equip Bowser with the Intruder Fangs to make it easier. While he loses 58 Power points by equipping the Intruder Fangs, the fact that he can attack twice per turn will make it easy for me to defeat enemies, especially the Dark Fawful Mechas. The fangs also save me time as I do not have to wait after the enemies attack me for my turn to counterattack.

It may take me a few days more to reach Level 99 for Bowswer.