Here is my own video guide I made recently for this achievement.
Very easy and simple, you just have to get it right. Sequence 10-3 is the only time you use the cannon. There will be platoons of Redcoats marching on your position. There are about eight in one platoon. When one of them stops, wait for the one behind it to catch up and aim slightly behind the first. BAM. Leave a comment if you have any questions.