Flaming coals are pretty common within this game. If however, your struggling to find an enemy to throw one at, load up the level The Chieftain Strikes found in The Desert of Kuneer. In the bottom middle of the map are two stands that hold the flaming coals. In this mission you can only use coals to kill the main enemy, The Chieftain. Fly over to the coal stands and use the A button to pick them up. Then follow the Chieftain and when you are locked onto him use the A button again to throw it at him. If hitting the Chieftain doesn't give you the achievement, when you get him to half health he'll call in his wingmen. Hit them with the flaming coals and that will definately work. It's as simply as that, you can't beat this level without getting this achievement.