Defeat the giant worm!
This is a simple achievement which is done in Mission 03. Here you must kill the giant worm that follows you throughout the plains of the level. There are 2 ways that you can defeat this thing.
Option 1 - when he opens his mouth to do the wind breathing attack shoot some rockets down his throat.
Option 2 - grab anything that shoots and unload on its orange pods.
The worm doesn't actually die, he respawns. So when the intense music stops playing and he disappears for about a minute, you have earned the achievement so leg it out of there before he comes back because when he does, he'll have full health.
Note: YOU MUST FINISH THE LEVEL to get the achievement.