There's a flamethrower on top of an L-shaped construction site building in southern-most Alconquin. I see no way to get it. I tried jumping with Formula R from the nearby Unique stunt but the speed is not enough to land on the roof. Does anyone know a way to get this weapon?
You need some Body Armor and Grenades. Next, you'll need either an Ambulance or a Burrito (which are plentiful).
When you get to this building then head to the inside part where you will find a large concrete pipe. Next, back the rear end of the vehicle against the north end of this pipe (like you're trying to hitch it). Then climb the vehicle and onto the pipe. You cannot use a car because you will sit too low to get onto the pipe.
Next, head to the other end of the pipe. While facing the building then drop a grenade behind you. When it explodes then you will be blasted up onto the building. Basically you Grenade jump onto the building. How far you throw the Grenade will determine how much Body Armor you lose. It is possible to lose very little Body Armor if you throw the Grenade far enough.
Then grab the weapon and simply drop back down again. Easy.