The giant moth on level 5 is so easy to kill, you have to do it. On the way up the mountainside, you will notice a laser rifle for a VS. make sure to kill all the small enemies and grab this laser gun, you can still grapple with this huge gun so get to the top. when you reach the top near the entrance to the cave, just wait for the moth to fly above you and shoot it's belly. It only takes like 3 shots to kill it. you have to finish the rest of the level to get the achievement. I think it's like 50 points but it only takes 2 minutes.
This is probably old news but I figured I would post it. I originally thought killing the moth was really hard and I didnt realize you can grapple with VS weapons in hand.