Make sure you gather as much thermal energy as you can. On your way up the mountain, destroy all of the Akrid hives and get the rocket launcher on the metal slab jutting out of the mountain. You can also get the large VS laser if you want a back up. Then, climb to the top of the mountain, but do not go into the cave. Instead, stand at the top and wait for the moth to fly over you. Aim at the orange weak spots and shoot the tail part with your rocket launcher about two times. After the tail falls off, do the same to the wings. A couple more weak spots will 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 unlock it after you complete the mission.