During memory 5 of sequence 3, you'll come across a plantation that has guards and alarm bells. Let the alarm bell guard notice you and ring the bell, then assassinate him. Naturally, finding any other plantation or warehouse with a bell works fine as well.Shooting the guard, performing a normal melee attack or doing anything else but an assassination will NOT work!
The easiest way to assassinate the guard is by doin a running assassination (hold
at the same time while you have swords or hidden blades activated).