First, gather up as much thermal energy as possible. On your way up the mountain make sure to destroy all of the Akrid hives. Get the rocket launcher on the metal slab jutting out of the mountain. You can also grab the large VS laser if you want a back up. Next, climb to the top of the mountain but do not go into the cave. Stand at the top and wait for the moth to fly over you. Aim at the orange weak spots then shoot the tail part with your rocket launcher about two times. After the tail falls off do the same to the wings. A few more weak spots should appear near the top of his head. These are easier to hit with a charged laser shot. When the moth crashes into the side of the mountain the achievement will not appear. However you will bet it after you complete the mission.