In my attempt to defeat the Level 7 boss in The Gauntlet challenge, I leveled up the Mario Bros. to Level 57, but even so, they were defeated twice. It turns out that the Level 7 boss (Bowser X) is more troublesome than I thought. Whenever I use a Special Attack, he will inhale it using his Vacuum ability preventing me from using it until he spits it out latter on. This prevents me from using powerful Special Attacks that inflict heavy damage on Bowser X, and forces me to use less powerful Special Attacks.

I may consider doing the bold step and level up the Mario Bros. to the maximum Level 99 if I continue to have problems defeating Bowser X. It will take several days, but I am within three levels from reaching Level 60.