This Seeking Tech guide will explain how you can beat Captain Shortfuse in Astro Bot. Use the following video as a reference:
For the first step, you need to head to the left or right section. Make sure to use your power-up to turn into a ball when the flamethrower gets close. Once you get to the mountain, use your power-up on its top end the first phase.
For the second phase, you need to head to the mountain in the opposite direction. The path will be littered with enemies, so you need to be more careful. You can avoid combat or get rid of them. You can turn into a ball to avoid taking a hit from the enemies and the flamethrower.
During the final phase, you need to defeat the volcanic mountain on the central platform. Once you get there, turn into a ball and then wait until the coast is clear. When you see an opening, defeat the enemy and then use the mountain to end the fight.