First complete the 'Joint Tower' area as Mario, then press R as Bowser.
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You can't do it with Mario and Luigi, but you can with Bowser. (You have to unlock it though) You press Y and then Y again when you're in the air. If you breathing fire, you need to press R. Then it should work. Hope I helped! :)
Press B to jump, then R to do the ground pound. Any form of jumping, whether its normal, double or triple, you can still do a ground pound. Hope this helps!
the final show down with bowser jr. is very similar to your past 2 battles with him. though, you are already aware that he drops bombs and is in a much bigger carrige. once a spiked ball is on a walk way lure him near it. once he is near it ground pound the walk way. this motion will send the spiked ball hurtling towards bowser jr. do this 3 times to beat him.
Ground pound on them.
Have Wario ground-pound it.