You will first come across the "Freeze Ray"
in the mission "The Shrunk Machine"
. 2 workers alongside an EDF soldier will give you the Freeze ray to help you clear out in the next room, but you can keep it with you as it'll have unlimited ammo, it just needs to recharge is all.
Simply shoot the ray at the enemy until he's frozen, then kill him any way you like whilst he's still frozen for the kill to count for the trophy. If you walk up to a frozen enemy it will tell you to press
to execute the alien, if you do this, you'll get towards the
too. Good luck!