Easy guide to get this achievement in minutes.

Make sure the weapon you have equipped is the lowest damage knuckles.

Simply load up Mission 5, Episode 5.

Progress for 1-2 minutes until you are inside the train station. Run in the direction of the assassination target and aggro enemies along the way. Climb on top of any of the train carriages and then simply wait for the enemies to climb up and follow you. On X Box controller, holding X button will melee the enemy and then kick them off the train. You may need to break their stance first, especially for the bigger enemies. Once you have kicked all the enemies off, reload checkpoint. You will only need to do this 3 or 4 times and will take less than 5 minutes.