He is slow but he has a long range of attack. The best moment to attack him is when he finishes his lunge attack. Be careful when attacking him when he's swinging his halberd. Also, keep circling him to his left. This will allow you to move past his shield and deal heavier damage to his back. Don't be greedy; its always safe to be sure. As long as you land hits and avoid his attacks, you should be able to defeat him.
Another strategy to defeat him involves NOT operating the levers that will raise the platforms in his area. Wait for him at the edge of the battle area and evade when he uses his thrust attack. When done correctly, he should fall down, instantly killing him.
Another option for beating this boss is to shoot him from the platform edge of the first lever... If you walk down the path some you can see him in the tower. Just pelt him with arrows until he turns to face you and lifts his shield. Then take out the three in the tower than the one knight just before his door. When you enter his door you'll only have to hit him one or two times and he will die.