There is an achievement that has confounded multiple IGN editors while playing Lost Planet. Since I earned it, I'll save you some headache and explain how you defeat (but cannot kill) the Giant Worm.

The Worm (and you will know him when you meet him) can never be killed (only temporarily defeated). Know that right off the bat. He has big green glowing weak spots that you must shoot. Keep in mind, the Worm has two ends and sometimes shows the head (he charges forward when the head is up) and his ass (when he fires missiles out his bum). You need to do damage on both ends.

The most important thing is to keep running across the snow plains until you are away from the small flying creatures. Once you are away from them, it is just you and the worm. Aside from doing damage and being annoying, the littly flying bastards often get in the way of rocket launcher attacks. So get clear from them and then fight the Worm.

There is no special animation when the worm is beaten. The only way you will know is that the music stops. You don't even have to have all his weak points destroyed to "defeat" him. However, in about two minutes, he returns completely healed. If the music has stopped, you have defeated the worm and you can stop fighting him.

When you defeat him you will not see an Achievement Unlock. Instead, you must complete the mission to get the achievement. This is why many at IGN spent significant time trying to defeat this beast again and again, not realizing the reward was already netted.