Goomboss

Goomboss is the boss that guards the key to Mario's door in the Rec Room. He is the king of the Goombas. Yoshi has to defeat him to save Mario.

How to Find Him
Goomboss is found in the Goomboss Battle level. The room for this level is behind the 8 star door connected to the Rec Room, so to enter this level and unlock Mario, you need at least 8 stars. In the level, the Goomboss fight is at the red X on the map. Get to Goomboss by going straight ahead until you find a beanstalk. Climb the beanstalk, then go over the long bridge, onto the treestumps and down the hole in the middle of the highest treestump. Along the way, there are a few goombas, piranha plants, and a lakitu flying around the beakstalk, as well as the patches of Toxic gas in the level. If you manage to get past these few obstacles, there is a healing heart near the entrance to the Goomboss fight, so you can enter the fight with full health.

How to Defeat Him
To defeat Goomboss, Yoshi has to run up behind him, swallow one of the goombas that follow Goomboss, lay an egg, and then throw it at him. You have to do this three times - after the first two hits, Goomboss will walk faster and spawn more goombas behind him, and after the third time you hit him, he gives up Mario's key.

If you're playing the level as Mario, Luigi or Wario, you have to hit or kick the goombas into Goomboss - this is slightly harder because you have to hit/kick the goombas at the right angle, but it's manageable.

TIP
When Goomboss starts to run faster, it can take a while to catch up to him. A shortcut to get behind Goomboss is to walk up to his face, then jump around him. You have to be careful of the purple gas in the centre of the fight level, but this shortcut can save a lot of time. It is easiest to do this shortcut as Yoshi or Luigi, because of their flutter and scuttle jumps letting them jump long distances.