Your first target is his legs. On each of Sapientia's four legs, there are faces. Hit them until they are destroyed. For this and the next part of the fight, he'll dive underwater and try to eat Bayonetta twice while surfacing, so make sure to dodge after a huge ripple of water appears underneath you. After the second attempt his fin will be visible and he'll jump out of the water and dive back in. He'll then jump out on all fours and return to trying to kill you. Surf up to his legs and destroy them one by one.Once all four are destroyed, you'll have to do a little interactive Climax sequence - after summoning Phantasmaraneae, you have to steer Sapientia to the demon. You either have to hold left or right and press X continously. (It'll let you know which way.) The direction will change up after a while, so make sure to pay attention. Also be aware that every now and then during this sequence, Sapientia will try to attack, so counter it whenever it happens. For the second part of the fight he has the same moves, but this time you have to get in close and attack his head. Once you get close, he'll try to slam his hand onto you or try to eat you, so make sure to back off for the latter attack. Just keep attacking his head until you get the option for a Climax. Do the same thing as before and the final part of the fight will begin. You'll be sucked into a whirlpool with Sapientia waiting at the bottom. First he'll pull out four tentacles and blow you all the way to the top of the area, so start making your way back down to him. Along the way he'll whip out his tentacles again and either shoot some lasers around in a circle or a tentacle will go underwater, pop up in front of you, and blast a laser at you. Once you get near Sapientia, he'll try to smash you with his hand and then either shoot a giant fireball at you or just try to eat you. Go in and attack him. After a few rounds of this attack pattern, he'll send you back to the top of the whirlpool. Repeat this entire process until he goes down for good. See: Bayonetta