His head. His heels.
First sprint past the Tower Knight up the stairs on the left or the right, and continue all the way around and kill all of the Crossbow men which line the walls on both sides of the court yard, then pick a method of attack as follows.
- The easiest way to get behind him is to run through his legs. If you try to circle around him he may get in a lucky sweep with his extremely large lance.
- If while running through his legs his shield starts to glow and he lifts it up horizontally just keep running through. He lunges forward with this attack so its very unlikely that it will hit you if you are behind him and you keep moving.
- Sometimes while running through his legs you can get pinned between his leg and his shield. Don't panic just keep running. He will eventually lift it up freeing you, and again as long as you're behind him his shield bash attacks won't reach you.
- After you've knocked him down and hit him in the head a few times, he does a quick vertical shield drop as soon as returning to his feet. So make sure you give him a little space, and again stay behind him.
- Finally, if during the fight he backs himself up against a wall, run to the opposite side of the arena and stay on the ground floor. He won't use his magic lance attack while you're there. He will just walk out towards you and away from the wall. If you don't do this he may accidentally step on you with just his regular walking motion. This wont do a ton of damage but it will temporarily stun you which can leave you open to one of his larger attacks. If you're stunned and backed against a corner, there aren't many escape routes.
- Bringing the spell Light Weapon will help tremendously if you plan to melee him. While Light Weapon is in effect you'll be doing damage to him even while attacking his feet (normally the Knight does not take damage from melee strikes if you attack his feet). Additionally, casting the Anti Magic Field will prevent him from using his magical lance attack. Once he's down, he'll take an extremely large amount of damage from your now enchanted weapon. In fact, w/ the Blueblood Sword pushing nearly 600 attack (stat), he took 2,500~ damage in only 3 swings, which was enough to take him down on his very first fall.
Video tutorial: Walkthrough
Sprint to get behind him and target the Knight's ankles. Be careful as he will use his shield and do a large area hit all around him. Keep circling around the back of him if you can with your own shield up. As long as you remain behind him, only his back step and leap back can damage you. These are his weakest attacks. Eventually, after enough damage is done, he will fall over and you can attack his head for massive damage. Either run forward through his legs, or to the side to avoid having him fall on of you.
Alternate Melee strategy - staircase shuffle
Using this method will take a long time but it's quite safe and simple.
Once you've killed all the archers head up either of the large staircases. When you see the Knight ready to throw his lance. Sprint down the stairs towards him and slash at his exposed foot. Then run back up the stairs again. You won't do a lot of damage but you shouldn't get hit either. The trick is to try and get him to move close to the bottom of the stairs so when he goes to throw his lance and you run down the stairs you'll hit him in his body and do greater damage. He will jump back every so often, so you'll need to move up and down the stairs to goad him forward. Occasionally he will move himself towards the other staircase. In which case run across to the other staircase and repeat as above.
Note: In NG+ and further, having a Dragon Sword +3 or a Magic weapon such as the Large Sword of Moonlight makes this method preferable since the Tower Knight normally doesn't have time to retaliate effectively and requires a lot less baiting. You can attack at least twice with larger weapons and more for smaller weapons and then either retreat up the same staircase or if more convenient, the other staircase and loop back around or use the same strategy from this staircase.
Bow or Magic
Video Tutorial: Ranged Strategy
Staying on the walls, go to the archway above the entrance. Attack the tower knight from here, using the stairs and railing for protection. He will use his spear to do a ranged attack, and when he does this move to shelter behind the railing. Then step out and prepare your attack. Release the attack the moment he moves his shield away to start his ranged attack power up, then side step behind the railing for protection, and repeat.
Note that you should be close enough to the railing to allow you to fire over it, but far enough back so that all of his attacks hit the railing not you. There is no need to leave Soul Form if you're wearing the Cling Ring.
Another good position to attack from a distance, is from the side, on the railing, close to the entrance, where the railing is the lowest. You can walk up to where the archer was, and when he tries to throw his spear, you fire at him. A magic attack is faster than his spear throwing, so you have time to move aside safely. After the spear hits the wall, repeat the process.
On each side of the arena walls are three small stairs with an archer on each of them. After you take out the archers, you can stay on the the first stair on the right wall ( as seen from the entrance ) during the whole fight without getting hit once. Probably you are safe on the other stairs as well.
You can also aim at his feet with Arrow/Magic and it will make them "spray" just like if you were hitting them melee. Doing this enough times will also make him fall and you can attack his head with magic or jump down and bash his head in. Enchanting your weapon will do the largest amounts of damage.
If you want to save a few Arrows or Spices, this is a good time to use any of the Throwing Knives you may have gathered. The Knives will only do 10 damage each when thrown at his head, but accumulated this can save Arrows and/or Spices. However the damage itself is minimal and throwing them can be time consuming. Since in NG+ Firebombs are close to useless in Boletarian Palace, these can be thrown using the same method since they fall short and hit the Tower Knight's feet and have a chance of releasing some gas.
Poison Cloud or Death Cloud
Tower Knight is weak to both Poison and Death Cloud and can be taken out easily with either, even if it takes some time. Death Cloud will paralyze the Tower Knight and after a few hits, you can simply walk up and swing away.
After killing the archers on the walls, get behind Tower Knight, lock-on to his head, hold down R1. Since you can keep moving while firing you can easily stay behind him and avoid all his attacks. Some attacks will hit his head doing extra damage, some will only hit his chest; doesn't matter since time is on your side.
Tower Knight is very weak to Soul Arrow and can be taken out with just a few of them. First go around and kill all the archers, then just lock on to the Tower Knight and strafe around him (being sure to dodge his forward shield bash), continue dodging and strike him with the Soul Arrow when he has left himself exposed. Also you can aim at his head to do maximum damage turning this into a relatively quick battle.