Bowser Jr's Lava Reactor: King Kaliente's Spicy Return

Planet #1
You start right next to a Launch Star encased in a glass cage. To free it, you'll have to lure a Bullet Bill from the other side of the back to the cage. It's easy to get lost, so we've come up with the most direct path to get it done.

Stand to the left of the caged Launch Star, then long-jump over the gap to the land strip.

After crossing the gap, simply follow the left edge up to the Bullet Bill launcher. Catch the attention of one of them, then run back down the way you just came and long jump back over to cage. Now quickly run behind or on top of the cage to force the Bullet Bill to ram into it, freeing the Launch Star.

Planet #2
The platforms will sink into the lava if you stand on them too long, so you gotta move fast (or keep jumping). Grab the Power-Up mushroom from the lava platform on the left, then continue to the far edge to deal with the two melon-spitting enemies. Kill them by rebounding the melons back at them with a spin attack, then use the Launch Star that appears.

Boss: King Kaliente

King Kaliente is back and has learned a few new tricks too. Thankfully, his weakness is the same: just spin attack the melons he spits back at him to deal damage. Like before, a mere 3 hits will get the job done. However, this time the battle takes place on a series of floating platforms, which means you can't stand in one place for too long, otherwise you'll meet a fiery demise. So either stay on the move, or jump in place to avoid sinking into the lava.

Aside from that, the King will launch a trio of flaming rocks between attacks which can easily be side-stepped. However, keep an eye out for the blue flames that roam the arena. Also, after the second hit, fireballs will rain in from the sky - as long as you stay away from the edge of the platforms, they can't harm you.