Not counting Quiet, the Man on Fire is the first boss you’ll encounter in Metal Gear Solid V: The Phantom Pain. He appears at the end of Mission 20: Voices, and you’ll have to defeat him before you can leave the hot zone by chopper. For this fight, I recommend bringing a grenade launcher if you have one. If not, a simple lethal pistol will do as well, but the grenade launcher makes things a little easier.
At the end of Mission 20, Skull Face will spot you in the hospital and call upon the Psycho Kid and the Man on Fire to attack you. You can’t fight him in the hospital, so just run out into the open space. Kaz will inform you that he can’t bring the chopper in until the Man on Fire is down, as he’ll just blow it up. As soon as the fight starts, look out for the two giant water tanks in the area. Your goal here is to lure the Man on Fire to where the water tank is, and then blow it up and douse him in water.
If you have a grenade launcher, all you have to do is shoot the water tank itself when the Man on Fire is in the area, and the water will fall on him. If not, use a pistol to shoot the red barrel at the base of the water tank, and that will do the trick as well. The hard part of this fight is getting onto the chopper before the Man on Fire gets back up. I suggest calling in the chopper right before luring him to the water tank, as this will give you a bit of time to get in and start firing with the gattling gun.
