In the challenge Node which is located in Bowser's Body, there is a mini game called "The
Gauntlet" You can't play the mini game at fist, but you can later.
In the Gauntlet, you face Bosses you've beaten before, but this time they are stronger.
You need to pay coins to challenge a Boss and it gives a recommendation to what level you
should be to defeat them. You also have a certain amount of turns to defeat the boss.
Finally, you get a prize of coins for completing each class.
Defeat these bosses the same way you did in their original form.
 Class 1: 
 Turns: 
 5
 Durmite X: 

 HP: 
 575
 Attack: 
 85
 Defense: 
 46
 Speed: 
 52
 Biffidus X: 

 HP: 
 32
 Attack: 
 26
 Defense: 
 22
 Speed: 
 74
 Class 2: 
 Turns: 
 15
 Alpha Kretin X: 

 HP: 
 135 (Each unit of it's body)
 Attack: 
 85
 Defense: 
 127
 Speed: 
 79
 Beta Kretin X: 

 HP: 
 810
 Attack: 
 85
 Defense: 
 76
 Speed: 
 76
 Class 3: 
 Turns: 
 18
 Wisdurm X: 

 HP: 
 2,380
 Attack: 
 156
 Defense: 
 127
 Speed: 
 79
 Class 4: 
 Turns: 
 10
 Bowser Memory MLX: 
 
 HP: 
 M-2,500 L-3,000
 Attack: 
 M-200 L-186
 Defense: 
 M-96 L-104
 Speed: 
 M-85 L-51
 Class 5: 
 Turns: 
 15
 Junker X: 

 HP: 
 3,080
 Attack: 
 179
 Defense: 
 129
 Speed: 
 123
 Junker Can X: 

 HP: 
 134
 Attack: 
 137
 Defense: 
 110
 Speed: 
 152
 Class 6: 
 Turns: 
 12
 Dark Star X: 

 HP: 
 3,993
 Attack: 
 304
 Defense: 
 164
 Speed: 
 135
 Dark Satellmite X: 

 HP: 
 416
 Attack: 
 180
 Defense: 
 127
 Speed: 
 176
 Class 7: 
 Turns: 
 35
 Durmite X: 
 Kretins X: 
 Wisdurm X: 
 Bowser Memory MLX: 
 Junker X: 
 Dark Star X: 
 Bowser X: 

 HP: 
 You're
 Attack: 
 out of luck
 Defense: 
 here.
 Speed: 
 Mariowiki didn't
 Prize: 
 list them :(
That's right. All of those bosses plus Bowser X are the bosses you have to fight for Class
7.
This Bowser isn't like the original one. so....
 New Attacks: 
~After Mario or Luigi uses a Special Move, Bowser will use his Vacuum move and that
Special Move cannot be used for 5 turns
~Bowser will grow large and randomly shoot Fire Balls everywhere. You must jump over some
and others you don't have to move at all. Then, Bowser will chase after the brothers and
both have to run away and jump over pillars of fire to reach two springboards which will
let you escape from Bowser and this ends his attack.
~Bowser will start to spit fire balls one at a time at the brothers. Counter them with
your hammer and Bowser will be on fire. Then, he'll roll up into his shell and start to
bounce around on the top screen and fireballs will rain down on the brothers. You must
counter the ones that are in reach of hitting Mario and Luigi with your hammer. Then
Bowser will try to ram into one of the brothers quickly. You must counter it. Then, he'll
try to go for the other brother and if he fails, the attack ends.
 Website: 
Mariowiki for stats and pics