I play on Expert aiming and its almost impossible to rescue him in time.
It was awsome on the first hanging I saw though. I couldn't rescue the guy but I killed the guys who hung him, looted them, shot down the dead guy and looted him, shot their horses in the head and skinned them and rode off --- Under a cold October sky, I wait I had dinner with CelticsV3!!!
Dead Eye is really the best way on Expert aiming, but if you notice when you aim, your aiming dot turns from gray to white when you are able to hit your target, so that gives you a bit of an idea of how to aim, seeing as you get a short amount of time to shoot him down.