Outward

During the third Faction Quest for the Blue Chamber Collective, you need to kill a Green Shell Horror, making this trophy part of natural progression through that Faction. Any Shell Horror counts for this trophy. For an easy kill, you can flip the first lever you see after entering the Cabal of wind Temple. The lever sets off an explosion on the floor below where a Shell Horror is walking around.

Note that when playing co-op, both players will unlock the trophy after killing a Shell Horror, irrespective of which player dealt the final blow.