Meet up with Haytham and his informant. After a chat to figure out the next step, follow Haytham as he leads you to where some of the Loyalist soldiers are stationed. Climb up to the top and get ready to fight. Haytham will start the fight without you so jump in whenever you are ready to help. You should be able to start the fight with a comfortable two kill assassination.
This fight is likely not as difficult as most you've had recently. There are a few grenadiers who you will have to fight and they will require you to mix it up a bit. Aside from them, the three officers will stay off to the side. They won't engage in the melee but they will still fire regularly from their position. There's a constraint for using human shields to avoid these attacks twice. Be sure to do this before you kill everyone and end the fight. When you've stopped all of the enemies, go up to the officers and take them prisoner.
Unfortunately, this will not go entirely smoothly and one of them will get away. You'll have to chase after him. He's not particularly fast but Red Coats will get in the way and try to block your pursuit. Just go around them however you can get close to him. When you get close, try to get up on a platform off to the side so you can perform your tackle from above to meet the constraint.
With the prisoner in hand, you'll take him to Haytham's stronghold. Once inside the fort, use the A/X button to push the prisoner forward. He will then walk a short distance and stop. When he does so, you need to push him again. Continue to do this until you reach your destination.
