You'll get a new blade from the bank vault near the end of the 3rd mission "The Bank Job"
This provides you with a new ability: Hold
to charge up your blade and launch a small ranged attack. This will knock enemies off of their feet. As you approach your carriage back to the Dreadful Whale, you will see two enemies standing right next to it.
Quick save, then charge up your attack and shoot it at enemy near the edge of the ledge. You want to knock them off using this.
If you can't pull it off without killing them beforehand or don't unlock the trophy, quickload and try again.