On assassins creed how do you kill sibrand

When your on the docks when he kills the priest there are 4 guards standing in front of a pier lure them away or kill them then get on the pier and keep jumping on the boats and those thick wooden things then jump on those towers and kill the archer sibrand owns the big ship and he has a helmet if you see arrows flying around its just him shooting randomly into the sea,when he turns around just jump on the back of the ship and a Assassinate him if he sees you he will run along the wall going back to the city but theres only about 10 guards on the wall and 2 on his ship.