Grand Star Battles
Bowser Jr.'s Robot Reactor (first on video)
Bowser's Star Reactor (1:40)
Bowser Jr.'s Airship Armada (2:40)
Bowser's Dark Matter Plant (3:50)
Bowser Jr.'s Lava Reactor (5:40)
Bowser's Galaxy Reactor (6:20, we didn't spoil the ending)

And that's it! Head to our forum  if you have further questions. Odds are someone there has tips on getting the 121st star...