After several days levelling-up the Mario Bros., I finally reached Level 99 today. It was a good thing that I levelled them up in the Airway section of Bowser's body and the fact that each level since Level 40 is 47,000 EXP, as it would have taken me more days to reach my goal. Even at Level 99, many enemies in the Airway are still undefeated whenever I use a pre-emptive attack. I think they have stronger Defense.

Since my first goal is finally completed, the next goal is leveling up Bowser to Level 99. He will be more challenging, as the foes he confront in Peach's Castle do not generate that much experience points as do the Mario Bros. in Airway. I will start tomorrow.