For this achievement you have to beat th first stage without dying. I would recommend sticking to the high ground and hit the mortar with the flame from the distance in the starting point. Then avoid the mines after the mortar. After this continue sticking to the top of the carriers. Once you have reached the end i recccomned you conserve all 3 of your shots for the end. I say this because in the end there are 3 waves coming from a truck. When an urarmed soldier (brown) is about to get you unleash your first flame. then when a second unarmed soldier is about to get you, repeat. And then for the 3rd close henchman unleash your final flame.