Instructions
Use fire based weaponry, as fire is Phalanx's weakness. Use Turpentine on your weapon, firebombs or fire-based spells to deal increased damage.
Create an opening on Phalanx by attacking one area repeatedly. The Phalanx's body is surronded by 35 Hoplites, and while some Hoplites separate from the body, most defend Phalanx with spears and shields. Focusing on one area and defeating the Hoplites create an opening to damage the Phalanx itself.
Attack the Hoplites, the small miniature versions of the Phalanx, from behind as they separate from the main body. Defeating them as they split off allows you to not be overwhelmed later on.
Attack the Phalanx directly once you create the opening. Over time, watch for the Hoplites to respawn.
Use one of the available pillars to hide behind to replenish your health and stamina if needed.
Continue to damage the Phalanx using the strategy above until you win the battle. Watch his health bar to know what attacks do the most damage and bring him down to zero.