Likely course of action for Heat man:
Fire heals him, as do explosions. They also make him more powerful. Please do not use fire and explosions on Heat man.
He's weak to Shadow Blade, Top Spin, Bubble Lead, and Air shooter in the game.
Oddly, he gives 0 shits about Metal Blade, despite his weakness to Shadow Blade. Presumably the Shadow Blade is hitting him in the back, where his temperature dial is.
Now, assume for a moment that Heat Man has his full retardation removed.
He will keep his flame shield active at all times.
He'll use his sensor to stop anyone from getting behind him.
As we don't have anyone immune to TWICE THE SURFACE OF THE SUN (seriously. It's 12,000C), we aren't getting in melee range of him, and any projectiles that can melt, will melt.
Gemini Laser does hurt him, but not very much, and if a Laser heats its target, it might heal or berserk him.
That leaves Bubble Lead and Air Shooter, both being fire extinguishers. Though if I know my fire, and Heat Man's been upgraded, it's likely that Air Shooter will just make him burn hotter.
Flood the area with Water, Flood the area with Mud, Flood the area with Liquid Nitrogen.
As Bullets and Lasers are likely ineffective, any heat-based weapon such as explosions or plasma will just heal him, and meleeing him is lolno, our only options are flooding the area or running.
If we can somehow store a shot of the Mikasa's 0 Kelvin cannon, it will oneshot him.