What I did was just get the right timing and press L1 whenever the clones attacked and whenever Zeus attackes with his lightning projectiles and chuck them right back at him. What I also did was stay close to Gaia's heart because if you do that, Zeus doesn't do his regen thingo when he charges towards Gaia's heart. Plus if you stick to Gaia's heart and attack the clones and Zeus with area attacks, you'll also damage Gaia's heart at the same time and so you'll get Green Orbs and replenish your health.
When I first played on God Mode, I took me a million times to beat Zeus because I didn't dodge or L1 much in the final battle. I actually kinda fluked it on God Mode.
Then when I played Titan Mode, since throughout the Titan Mode I had to block with L1 so many times, I got really used to it and then it took me a few times to defeat Zeus in the final battle but once I got the hang of it again, during the play when I defeated Zeus, my health didn't even go below half.
Dodging, staying close to Gaia's heart and pressing L1 in the right moments is key to Zeus' final battle.