Central Boston will have been liberated as part of the main storyline during sequence 6.
Recruitment Mission - Gangs of Boston
After completing enough of the liberation side-missions in North Boston, you will be able to receive a mission from Duncan Little at the contact objective marker.
You will be tasked with killing the templar leader . He is located at the end of the northernmost dock in Boston. Fight your way through with the help of your assassin recruits. Once the templar is dead a scene will play to complete the mission. At this point, Duncan Little will have joined your ranks as an assassin and you will earn the assassin ability - Bodyguard .
- Chase and kill a military pursuer. When you find these guys, they will be mounted and chasing another person riding away on a horse. Kill the mounted enemy either by chasing and taking him down with your hidden blades or even easier - shoot him.
- Eliminate a conscripted prisoner’s escort. As you wander the area you will find groups of 3-4 guards escorting two friendlies about. Kill the guards to free the conscripts and complete the task.
Recruitment Mission - Martial Law
After completing enough of the liberation side-missions in South Boston, you will be able to receive a mission from Clipper Wilkinson sitting on a bench outside a house marked with an liberation objective marker.
You will be tasked with killing the templar leader . When you start the mission, you will be facing him. Simply go into your assassin moves wheel, select the marksman ability and target the templar. Your assassin will snipe him, as he dies a scene will play to signify mission completion.
At this point, Clipper Wilkinson will have joined your ranks as an assassin and you will earn the assassin ability - Marksman .