Once the mission starts you will need to head over to the water tower with Pat, kill any enemies that you see on the way there and then climb up to the top of the tower. Next use the Sniper Rifle to shoot the rope around Soapys neck before he dies, once he is free use your rifle to kill any enemies that try to kill Soapy, and soon some enemies will appear on top of the building across from you, use your rifle to kill them quickly. Next some gang members will come on horses so head down to Soapy and kill them with whatever you see fit. When they are all dead get on a horse and follow Soapy to the dock, killing any enemies on the way, once you get there talk to the Ferry Operator to begin crossing the river. Use your sniper rifle to kill the enemies that appear on the cliffs and then use your normal rifle to shoot the rope once you reach the other side, this will cause the other ferry to drift away down a waterfall. Once everyone is dead the achievement will unlock.