This page offers a guide on the boss galaxy of the Engine dome. Bowser Jr.’s Lava Reactor is unlocked using 45 Power Stars.
Guide the Bullet Bill
On this crumbling planet you’ll need to jump over the cracks (and avoid the blackhole!) and get to the Bullet Bill shooter. You’ll then need to guide the Bullet Bill towards the caged Launch Star which was near where you began on the planet. There are other cages on the planet filled with various things, and you can choose to smash these ones too but to progress you need to use the Star Launcher.
Shoot the Octopi
In this next section of the level, you’ll be reintroduced to the concept of counter attacking the Octopi’s coconut attacks by spinning them back at them. You’ll need to do this to both of the Octopus enemies to spawn in a Star Launcher that’ll take you to the final section of this level. Watch out for the floor as the separate hexes slowly sink into the lava the longer you stand on them. You’ll have to keep on moving to make sure you don’t fall in!
King Kaliente’s Return
King Kaliente is back! And this time the fight is going to be tougher! Like the previous boss fight with Kaliente, he will still throw coconuts at you, but this time he’s prepared for your counter attacks. When you spin a coconut at him, he will deflect it, send it back to you. You’ll need to spin attack the same coconut at him twice in order to get some damage in. You’ll also discover that the floor is very unstable, you can’t stand on the hexes for too long at a time as they’ll sink into the lava. So make sure you keep moving around to avoid falling into the lava.
You’ll need to damage him three times to defeat him. During the fight he will shoot flaming coconuts and spawn in meteors and lava bubbles. When you’ve hit him two times, he will enter his “angry” phase where he spawns in more meteors and lava bubbles and shoots out more flaming coconuts. You will also need to counterattack the coconut three times to get the final bit of damage in. This can be tricky, especially since you need to dodge all of the incoming enemies and meteors, and watch out for the sinking platforms.
When you’ve hit him three times, a Grand Star will appear upon his defeat!