At the beginning of the fight you'll be on top of Gaia's head with two of the horse-crabs (Hippocampi) grabbing ahold of her. You need to hit both of them until they let go while also avoiding Poseidon's attacks. He alternates between casting lightning on half of Gaia's head and commanding his Hippocampi to attack. Once you free her, the next portion of the battle will begin.

In this section Gaia has ahold of Poseidon and you must attack him until you see the  above his head. He has four attacks he wil use in this phase: a fist slam, a backhand, he will slam his trident into Gaia's hand and surge lightning through it, and for the last attack he will pick up and slam his trident down three times going from left to right. Once you see the  above his head, climb the wall on the right and perform the QTE.

The third section is exactly the same as the first, with the Hippocampi holding Gaia's head. The last section is just a long QTE.  Once you complete that you will unlock this trophy.