I had an interesting technique for the ropebeater, that is the one you're talking about right?
I just equip my hidden blades then hit him with it, switch to knuckles and then you can actually hit him.
Then do it again, im not sure if the hidden blade itself does actually reduce his health.
I was having a rough go with the ropebeater but I figured out a solid strategy that will have him defeated with little to no trouble. Deal with his mooks first then get him between you and the low rail where you first see him. When you attack Connor will kick him over the rail doing damage. It will only take about 4 hits and he'll drop.
There definitely a glitch on the "Sailor" fight on the ship. I can beat the three others and I can get the sailors health bar down to the red, but after that, I can't defeat him. I have hit him as much as 6 or 7 times after he is in the red and he won' t go down. Anyone have an idea to bypass this so I can finish the mission?