Instructions
Use your weapons to chop off the legs of the Hippocamp. Start with either the left or right leg using combos to chop off the legs. Remember, Poseidon will use his trident, thunderbolts and wind attacks as you try to chop the legs off so be sure to dodge them as you're removing the legs.
Use the Hippocamp's arm as a bridge to get to Poseidon. Aim at Poseidon's chest and use a combination of blade and magic attacks. While you are attacking, be sure to avoid his trident by constantly moving.
Look for the circle image above his head. This is an indicator to press "Circle" and prepare for a QTE sequence (button pressing). When prompted, press "Triangle," "X," "Circle" and "Square" in order. If you press the wrong button, you will fall off his head and be forced to hit him until the QTE sequence reappears.
Chop off the Hippocamp's legs once again. Use the arms of the Hippocamp to make your way toward Poseidon. Again, use a combination of blade and magic attacks aimed at Poseidon's chest while avoiding his attacks. Press "Circle" to enter the new QTE sequence and press "R1" followed by "X" to defeat Poseidon.