Zeus in the Gaia's heart room. It's incredibly easy, although this strategy may make the fight last a bit longer. Switch to the last weapon (forgot the name..the green chain thingy from Hephastus. Just spam L1+O. He can hardly get out of it and break your combo. Just make sure you always go for Gaia's heart when it glows to stop Zeus from getting his health back. Sometimes if he gets too many clones going, run away and wait for the very last slam drop the real Zeus does. You can't block it or parry it. You just have to be off the ground when it goes off. Then go right back into spamming L1+O. If you get completely surrounded and they keep breaking the L1+O chain, just throw down some magic which is a long lasting shock. That should take care of the clones. Then go back to L1+O spam. Rinse and repeat as necessary.
I hope that helps someone.