I myself would suggest using Watt as a partner, Mega Smash, Feeling Fine, Zap Tap, and Happy Heart/Flower badges, and have one or two Life Shrooms.
Just keep using Mega Smash and Electro Dash, plus block Bowser's light wave attack if you can. If he kills you once and the Life Shroom goes, then use a Deluxe Feast or Jelly Ultra. When Mega Smash runs out, use Star Storm, then Shooting Stars.
Of course, this is my strategy and a lot of other ones will work too.
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