To unlock the Eye Witness trophy in Assassin's Creed III Remastered, you will need to witness a predator killing a soldier.
The best place to do this is in the upper left corner of the Frontier. There will always be a guard standing on top of a rock with his back to you. Sneak up behind the soldier and place some bait on the rock, and then hide in some branches nearby so that you can still see him.
Wait a short time and a Lynx will be attracted to the bait. Wait for the predator to kill the soldier and the trophy will unlock.
You can view a video below (credit to 360GameTV on YouTube):