During the second level, Hard Landing, you will be required to pick up theM2 Flamethrower from the corpse of one of your fellow soldiers and burn the Japanese soldiers out of the bunker which is situated in front of you. After, you and you your fellow soldiers will head towards the airfield. During this journey, you will see a large patch of thick grass on the right of the path. Approach the grass with the Flamethrower in hand and some Japanese banzai soldiers will come out of hiding. Aim towards the nearby tree and press , the tree should burn and the achievement will unlock.