I am currently at the halfway point in my goal of leveling up Bowser to Level 90 after reaching Level 85. Every new level, Bowser gets more powerful to where in many cases, he can defeat enemies without them even countering him. This means that battles are getting shorter and will take me less time to level up. I am also using Bowser's Spiked Ball ability to quickly move around Peach's Castle and save time.

This evening, five more levels.

Unrelated to Bowser's leveling up, I was able to clear The Gauntlet's Level 7 within 11 turns. I defeated the previous bosses in nine turns and defeated Bowser X in two. I do not know whether I can defeat him in a single turn as I estimated that he has about 6,000 HP and the Mario Bros. need to use Falling Stars special attack (the most powerful) twice to do that. Unfortunately, Bowser X inhales the Falling Stars icon after it is used, so I cannot use it again until the next several turns when he vomits it out of his stomach.

Regardless of the trouble, clearing Level 7 within 11 turns was possible due to the Mario Bros. leveling up to Level 99.