The West Point Payback trophy in Assassin's Creed III Remastered is story-related and can't be missed. You will unlock the trophy when you complete the Justice Served mission in The Tyranny of King Washington: Infamy DLC.
During the mission, you will have to follow Israel Putnam, release a bear from a cage and then infiltrating the fort.
Once you have released the bear, use the Wolf Clock by pressing
to sneak inside the fort and make your way to the orange circle on your map. If you are noticed, you will desynchronise and have to continue from the last checkpoint, so be careful not to get spotted.
Once you are at the circle use your Eagle Vision. Arnold will be highlighted as a gold target. Use your Wolf Cloak again, run over to him assassinate him with your hidden blade by pressing
.