This trophy can be obtained early in the mission.
The creatures are known as Seekers (Stealth and Suffocation are the key features to look out for). Two Seekers will make an appearance when you taken down a second but "larger" UFO ship.
When choosing a squad, I'd recommend having at least two Assault (Shotgun) members and two Support (Assault Rifle) Members.
*Save your mission at the start, that way you know where the Seekers are located the next time you mess up.*
Make sure your team are together, but not too bunched up.
Once you discovered Two Seekers, they'll go into stealth mode. Immediately, make everyone go into "Overwatch", because there will be a guarantee the seekers will begin to strangle a soldier once your turn is over.
Once exposed, the soldiers will react to their shots, if they succeed you'll earn a trophy.
If the Overwatch tactic didn't work, then use one of your "Assault Class soldier" to go up close and shoot the Seeker (Shotgun: The closer, the more damage) and it should take down the seeker or release the suffocating soldier depending on the damage it has taken and that will also earn a trophy.