This guy is really tough. Hit him enough times in the chest and you'll knock him off. Shotgun is hard to aim.
Regular pistols are okay, uzis are better.
Remember when you knock him off his horse, get in his face quick with uzis, else he'll get back on his horse. Now go open the gate, pull the rope, and make your way inside one of the rooms with the swinging chains. Flip up the floor, go down and take it from there. Practice adding health while still holding on to the pole. Regular pistols are okay, uzis are better.