Use the Magnet Gun to fling debris BACK at a Tentacle.

Tentacles are long worm like aliens that pop out of the ground, they pick up debris and throw it at you and can also hit you at close range with a stomp attack.
This achievement is simple to obtain and the 1st place you can get it is after you are arrested by the red faction and your convoy gets attacked. You will receive the order " destroy the pods"

As you come out of the cut scene behind you lies an upgrade station and a weapons locker. Grab the magnet gun and run down the hill to the left, at the bottom of the hill a tentacle will spawn.

It will pick debris from the building to its left, as it goes to pick it up aim at its head and then when the cursor goes yellow fire. You then wait until it chucks the debris at you and aim back at the tentacle and fire when the cursor is blue.

---

---