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 coming out of the mountain. You can also grab 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. 
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. Once you defeat the 
giant moth he will crash into the side of the mountain. Note: The achievement will 
not appear until you complete the mission.