When fighting the Stygian, you can max out your Chaoseater Blade by attacking it repeatedly. Begin the battle with the Stygian by killing phantom guard soldiers while riding Ruin. After the Stygian frees itself, ride along side the Stygian and strike its body. Do not strike the helmet or it will break off and the Stygian will attack. Equip Combat Lore to increase the experience you get from each strike; Bloodthirt will replenish your health. The Stygian will sink under the sand and resurface. Repeat the process to get the maxed out Chaoseater.
If you mess up and knock off the helmet, you can exit the battle. After removing the Stygian's helmet, ride Ruin to the closed doorway during the battle. You will jump off Ruin and have control of Ruin. Keep your back against the door and wait for the Stygian to come right at you. He will knock you right through the door and you can go back to Vulgrim. Come back to fight the phantom guard soldiers and the Stygian when you are ready.