Alternatively, you can also place a C4 explosive near the tank at the shallow pool. Once the Man on Fire wanders in between the tank and pool, blowing the tank up will cause an explosion that forces him into the pool, leaving him incapacitated for a short while.
Don’t bother shooting the Man on Fire with the grenade launcher, as he’ll just absorb the explosion. Also, if you have the water pistol, don’t bother using it against him either. It’ll take about 50 shots with the water pistol to take him down and frankly, it’ll take too much time and will just make it harder for you to survive the fight.
Once the chopper comes in and the Man on Fire is down, quickly hop into the chopper and start shooting at him with the gattling gun. If, for some reason, you are unable to get on the chopper even after using both water tanks, all you have to do is survive until it starts raining, and the boss fight will automatically end. The Man on Fire and the Psycho Kid will disappear, leaving you free to call in your chopper. However, do note that this method will probably rob you of that much coveted S Rank.