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.