Call of Duty: World at War

The easiest mission to attempt this on is Burn 'em Out on Recruit difficulty. You start the level with the flamethrower, just keep it out for the entire mission. Remember that melee and grenade usage is fine. Obviously the flamethrower will be your main weapon here, it has much better range than one would expect. Remember to go easy on the trigger with it because when it overheats it won't fire for several seconds, which you can't have when its the only weapon you're allowed to use. Only hold the flame if you're making a sweeping motion across multiple enemies, otherwise pulse the trigger. Just take your time and gradually advance through the level. Run out of cover, light some enemies up then retreat and heal. There are plenty of grenades in this mission so don't hesitate to use those for medium to long range targets or to clear out crowded areas.