Johnson's Errand is a next quest from the main storyline in AC 3 Remastered. The section below also contains the descriptions for optional objectives found during the task.
After speaking with Johnson, leave the inn and speak with Hickey beside the building.
Optional objective: Kill 10 mercenaries using firearms.
Soon enough you will have to shoot one of the enemies with the secondary attack button. Your main objective is retrieving the stolen chest and optionally killing 10 enemies with firearms. In order to complete the optional objective, always begin with shooting your pistol and try reloading as fast as possible so that you can shoot again before enemies can get nearby you.
After eliminating the first target, climb onto the wall before you and shoot another enemy on the left. Afterwards you should target the group of soldiers below. Two companions will aid you in this fight.
With the area cleared, blow up the nearby gate by shooting the gunpowder barrels on the cart.
Inside the fort you will find the chest you're looking for.
As you approach it, more mercenaries will appear. Shoot the first one and finish off the others with your sword.
After defeating the group, you should receive another objective - you can't let your companions lose more than 50% of their life points. Before leaving the fort, collect items lying by the enemy bodies. For that you need to hold down the interaction button.
Optional objective: Do not let Hickey or Lee lose more than half their health
Optional objective: Complete all optional objectives during a single playthrough.
Head to the marked destination with your companions. From time to time you will be attacked by mercenaries. You should always begin with shooting your pistol and then eliminating the other enemies in melee combat.
Your last target should be the soldiers shooting from roofs of the nearby buildings. After you eliminate all the other enemies, they shouldn't be too much of a threat for Lee and Hickey who will be hidden behind covers.
After reaching the designated spot, the mission will come to an end.