This page offers an overview on the boss galaxy of the Engine dome. Bowser Jr.’s Lava Reactor is unlocked using 45 Power Stars.
Bullet Bills - Bullet Bills will lock onto you and fly towards you if they spot you. The best way to defeat them in this level is to guide them into flying towards the cages to explode them.
Goombas/Mini Goombas - Goombas will simply run at you and try to damage you by touching Mario. To defeat them, spin attack them then run at them to kick them away, or just jump on their head.
Lava Bubbles - The Lava Bubbles will appear during the King Kaliente Boss fight. The only way to defeat them is to shoot Star Bits at them. They’ll follow Mario around and try to burn him.
Octopi - The Octopi in this level will throw coconuts at you. You can spin attack the coconuts so that they fly back at the octopi.
Meteors - Meteors are just big flying boulders that King Kaliente will spawn during the boss fight. You cannot defeat them, but they will crash and explode upon impact with the ground, so you just need to avoid them.
Boss - King Kaliente
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.