1) You have to be in the mission Derelict Reclaimed - This is the Mission 10.
2) You have destroyed two AA Guns already and your current objective is:
3) Arrive at the area where the 3rd AA Gun is:
4) Look to your right. There is a shack/container there with a bunch of weapons, ammo, and armor:
That container also has the M5 RPG Launcher needed to destroy the AA Gun.
5) Enter the Container and find the Flamethrower in the far left corner:
Enjoy your new legendary flame thrower.