The Flamethrower can be found under the Specials menu when at a shop for a cost of $2400 or you can wait for it to become free by unlocking more Radio Towers. If you have the money, buy it and if you have any spare I'd purchase the upgrade for it as well. Now all you need to do is kill 50 enemies with it. The weapon itself is pretty good but you need to get in nice and close for it to really work. Though once you do set someone alight with it, it'll kill them in no time. One burst should do the job.
This is best used on Outposts. Let the alarm ring out, and you'll have a new group of enemies to kill. Take note that this only works on enemy NPCs. Killing animals does not add to the counter but you can provoke your Rakyat friends to attack you and doing so will then make them count as kills. To see how many kills you've earned, head into your Handbook and press to see your Statistics. Scroll down to Kills and you'll find it there.