If you had missed your chance grabbing this trophy during Chapter 1/throughout chapter 2, then you will have a great chance at grabbing it during the mission "Battleship - Orick. California" which you get to (avoiding spoilers as much) by entering the enemies ship later on in Chapter 2.
You will eventually after a lot of walking/killing come to a bridge like thing, with an infinite spawn of chimera soldiers/one big guy down below, and on this bridge will be two chimera using augers, take one of their augers, and the ammo of the over one, now whilst you're on said bridge, look down at the enemies, and you should see a see-through cover infront of you, which fortunately counts as solid matter, and allows you to shoot at them, without being shot at.
After killing 50, or however many you need through this see-through cover, the trophy will be yours, try and preserve your ammo as much, seeming there's an infinite spawn of enemies so they won't be going anywhere.