You can unlock the Aftershock trophy in Assassin's Creed III Remastered during The Tyranny of King Washington: Redemption DLC.
During the second mission, you will unlock the Bear Might power and have to complete a tutorial that will show you how to use it. You will then be attacked by 30 George Washington clones. Keep grouping them together and use the power to take out as many as possible at once.
You can then reload the checkpoint and repeat as many times as needed until the trophy unlocks.