At times, the tentacle will pick up a piece of debris, and due to you shooting it or some other disruption, it will drop the debris on the ground. This debris will satisfy the requirements for the achievement -- so if you see a piece of a building that was dropped by a tentacle, use your magnet gun, shooting the debris first and the tentacle second. I found it a bit frustrating to try to shoot the debris while the tentacle had picked it up, as the auto-aim was a bit off-center.
You will have many opportunities to deal with tentacles throughout the game, but if you are farming this, the surface area is a good spot due to the space you have to maneuver.