Toward the end of The Tyranny of King Washington, Part 1: Infamy, you will be tasked with following Israel Putnam to a fort. Once there, you'll be tasked with releasing a bear from a cage and then infiltrating the fort.
Inside the fort are a ton of guards and soldiers, but your only two goals are to avoid detection and find and assassinate Benedict Arnold. If you're detected, you'll immediately be desynchronized and sent back to the last checkpoint. Make liberal use of the wolf cloak, summon your wolf pack whenever available, and above all else, be patient! This is a fairly easy kill, but you can't just blunder your way through to it.
Benedict Arnold will be gold in your eagle vision, but he'll also have a marker floating above his head. Once you have him in your sight, you can easily use the wolf cloak and give Arnold a stabbing; that's what I did. Summoning the wolf pack and shooting the target with the pistol or bow would probably work, too, but this kill is really set up for the wolf cloak. Just remember to keep an eye on your health.
Happy hunting!