Bring a full compliment of throwing daggers. This will make your job much easier.
To kill Sibrand, refer to the information you should have obtained via information gathering. The northern docks are devoid of guards. Make your way along the docks, ignoring the drunks and dockworkers until you spy a platform with a guard. Toss a dagger at the guard and hop across the poles leading to the lighthouse.
Once at the lighthouse kill the three or four guards and toss another dagger at the archer on the platform nearby. Make your way across the poles until you get to Sibrand's ship. Dispose of Sibrand in whatever manner you deem worthy, and return to the Assassin's Bureau to get your achievement.