You must first destroy his armor, which is the white bar at the bottom of his heath bar. Equip your spear and use Blood Drunk. You only have a limited amount of time before he calls other Immortals, so try getting his armor down as much as you can before he calls them. Make sure to block his attacks as much as possible, and run away when his razor hands turn blue or red as you cannot block those attacks. If he calls other Immortals, ignore the Immortal King for a moment and kill his friends. It is best to kill them with dual swords. After you destroy his armor he will jump on top of a cage and shoot his hands at you. You cannot block his hands. Run behind the pole with some dead bodies attached to it that is close to the cage he is standing on. When he shoots his hand at you, it will get stuck to the pole. Immediately run to another pole that is beside the one you were just hiding behind. He will shoot his other hand at you and it will also get stuck. Once this happens, run into the open and throw a spear at him (hold Circle + Square + X). It will go directly into his chest and do alot of damage. Keep on repeating this process three or four times until he is dead.
To easily defeat Giant Immortals, use a spear to break their armor first, then attack them with whatever weapon desired. The gray bar is their armor amount. When fighting the second Giant Immortal, he will run away and start to regain health. Go to the statue he is beside, then push it on top of him to eventually defeat him.
Use the following trick to defeat Mordonius for the first time. Mordonius is a large muscular opponent with a massive double-sided spear. You must be careful when fighting Mordonius, as he is very strong and can do a lot of damage, even with the best shield and armor. To defeat him, just throw spears at him. When you are first facing him there will be about four spears laying around. Grab a spear and hurl it at him. It is best to throw it at him from a bit of a distance while he is doing his special (when his weapon turns blue or red). He will almost always block the spears, but when he gets hit it will take out about one fourth of his armor. If there are no more spears to pick up, simply run around until a spear in the middle of the battle field pops up. When arrows are fired at you, make sure you are not close to Mardonius while you begin to block. Sometimes when the arrows are about to hit you, his weapon will turn red and he will knock your shield off. You will not be able to block the arrows, and they will either kill you or do a lot of damage.
-While facing Mordonius, you will be Dilios instead of Leonardas. While he is charging at you with his rhino, equip your spear and hurl it at the rhino Mordonius is on right before he hits you. The rhino will be dead but Mordonius will not be. He will kill his rhino then jump on to the top of a building. Go up the ladder and you will be facing him again. Simply hurl spears at him. One by the wall will come back after awhile if you use it, and he is still very strong. Simply run until you get another spear. After awhile of throwing spears he will be still alive and jump to a higher platform. Go to the ladder again and you will be on the same platform as him. He will run away from you, scared. Follow him until you reach a dead end then fight him again. Use dual swords to take him out.