Head out to the Frontier and make your way to Monmouth. When you get there, you'll find that Charles Lee has been causing trouble around here. The Red Coats will appear around the corner and attack. You need to cover the retreat of your allies.
You will be surrounded by patriot forces and you will take control of a cannon. You need to use the cannon to hold off the advancing forces. While the groups advancing from the front will likely be obvious, you also need to watch for those advancing up the right side. Quite a few of them can build up and start causing trouble.
Try and line up your shots and center the enemy platoons in the circle. Always shoot for the platoons with the red marker because these platoons are currently firing at you. As you go, if you fail to take out the entire platoon but have left only an enemy or two, change targets and go for a full platoon. Your allies will be able to take out the individual enemy themselves so you can use your cannon to maximum effect. You need to survive until the timer runs out so try to take out as many enemies as you can to keep from receiving too much damage.
Remember that you've got two constraints here. You need to destroy two platoons with a single shot and get rid of eight platoons with a single shot each. Try and take out platoons that are far apart and let two platoons pull up side by side so it will be easier to destroy both with one shot.
When you've survived, you will have to instead retreat. You will have three minutes to make your retreat all the way back to your allied forces. Along the way, however, will be allies that have been taken prisoner and are about to be executed. Watch out for the blue circles/white rings symbols on your map that mark your troubled allies as you make your retreat. At the very start, a horse will find you. You don't need a horse to make it in time but it's going to be very close (seconds) if you don't.
The first ally will be in the first town you pass. As you go through the town, you will find them in the town center by a tree, waiting to shoot your ally. You only need to attack them to create a distraction. Once your ally has escaped (and it has been marked off on your log in the upper left), you can actually stop the combat, get back on your horse, and ride away. Only do this if you feel confident in your escape because you have the time to kill these enemies and still make it in time.
The next execution will be just a little further up the road along the outside of a house on the left side of the road you are on. Kill them and continue up the road. After passing a field on the left, you should see the last execution about to happen just off to the right. Finish your sprint for the finish line to complete the memory.
