The Hidden Blade Assassin's Creed in Depth

This exploit is one of two ways to reach the guards in the same area as the target before he spawns there.

Alternate Route

You cannot approach the British army directly without getting shot at and eventually desynching. This is because you cross an invisible line, triggering that scripting. However, you can bypass that line by veering to the right, past the memory wall. Have to be careful because you're almost far enough from the wall to desynchronize. After dropping off the cliff you can charge straight into the British troops.

Have fun taking out Pitcairn's guards in whatever way you want. There are just a few high levels guards to deal with, nothing a rope dart won't take care of. To make the target spawn, go the one of the checkpoints to progress.

Easy Fight

With no guards around, you are free to kill the target in more creative ways. I found that the ax special move doesn't kill him, but merely does damage and knocks John Pitcairn to the ground. The tradeoff is that this move is unblockable. The blunt weapon special move is less effective because the animation involves throwing the weapon, meaning you have to constantly pick it back up. The rope dart also does a little damage every time you use it to pull him to the ground.