To unlock the Circus Act trophy in Assassin's Creed III Remastered, you will need to play through the story until you reach Sequence 10-3.
During the mission, you will take control of a cannon, and you will have to defend your position against groups of Redcoats who will be marching towards you. There are 8 enemies in each group. Once they stop, wait until the second group of Redcoats approach from behind them, and then aim your shot slightly behind the first group to take out all 16.
You can see the trophy being unlocked in the video below (credit to JuicyBoxMan on YouTube):