Select the mission Payback and work your way through the level until you are at the point after the Hind battle.
(You can get the 'With Extreme Prejudice' achievement during the Hind battle, if you dont already have it, by using just rockets.)
As you try to enter the cave an enemy with the AK47 Flamethrower tries to flame you. Kill him and take his weapon.
The less enemies you have to kill to get to him the better, so you have more to torch.
When going for this make sure you really flame each enemy.
If you set one alight and leave him to die, your teammates may shoot him and this is not a kill for you.
Just concentrate on singeing 10 enemies to a crisp instead of trying to 'lightly char' everyone.