1) Load up the third mission called "Hard Landing" on either solo or co-op campaign.
2) Play through the mission until you are required to pick up the M2 Flamethrower.
3) Pick it up and complete the objective.
4) You will begin to head towards an airfield with your squad.
5) You will see a field with long grass.
6) When you see this field, make sure you have your flamethrower out.
7) Walk towards the grass and Banzai soldiers will jump up and run at you.
8) Quickly burn them and the trees above them.
9) Some enemies will fall out of the trees as the leaves burn them.
10) The achievement will pop up seconds later.

Here is a video tutorial: