The goal to this achievement is to use the Magnet Gun to fling debris back at a Tentacle. This achievement is a piece of cake, I know you can do it to after you read this below:

1. Just as when your defending the red faction, there are weapons that are available for you to use.

2. Tentacles are huge aliens that are, well, a tentacle in form. I know you know what a tentacle looks like, but for those few who don’t, those are the long fingers of an octopus. Here in the game, they burst out of the ground and throw debris at you.

3. So when a Tentacle picks up a piece of debris, aim at the debris with the Magnet Gun and attach an anchor to it. As soon as the Tentacle throws the debris, attach the attractor to the Tentacle and the debris will fly straight back towards him.

4. As soon as you kill the tentacle, the achievement will pop out.

Below is the video version of this guide: