To beat Plooty and Rookie (Rookie is really a brain-washed Bowser with a mask), here is what you have to do.
It may seem like a good idea at first, but don't attack Plooty directly. If you do Rookie(Bowser) is gonna get really mad
Always attack Rookie because he is way more dangerous than Plooty. Use alot of Bros. Attacks to beat him fast.
Don't let Plooty steal Mushrooms from you, or he'll give them to Rookie and it will take FOREVER (and both Mario and Luigi will be dead by then) to beat them. If the icons tell you to use your hammer to defend, he is going to steal from you, but don't let it happen.
Eventually, Rookie will say he is getting a flame in his gut and he will start breathing fire, but you can jump over the flames easy.
Once you beat Rookie, all you have to do is wait. Plooty will attack once and he will run away with Rookie.