You will receive the next mission after speaking with Achilles. After 6 months pass, head to the designated spot on the Frontier and meet with the member of your tribe.
Optional objective: Reach Johnson undetected.
Your mission will be reaching the Indians on the top of the rock. For full synchronization, you cannot be detected.
Start off by jumping down the branches to the shore of the river, swim to the other side and head out as far to the right as possible while looking out for the soldier patrolling the area.
Further to the right there will be a part between some rocks.
At its end, climb the fallen trunk and reach to the other side of the precipice.
From there you will have to get even higher, to the second tree.
Cross it and head further until you see a crack leading to the top.
Two guards will be waiting there. Kill them both from behind and go round the two next patrols from the right, heading towards the marked spot.
As you reach the big manor, a short cinematic will start.
Optional objective: Limit Native American casualties - 0.
Optional objective: Completed all the constraints in one playthrough.
Afterwards you will have to climb the roof of the building and kill Johnson from above. use the nearby trees for that and avoid the enemies on your way.
Perform the task as fast as possible, as you won't be able to achieve full synchronization if any of the Indians die.
Optional objective: Perform a swan dive to escape.
With the enemy dead on the ground, run towards the river and slightly to the left. There you will find a rock from which you will be able to jump into the water and therefore complete the last optional objective.
Afterwards leave the red zone and the mission will end.