I'm one of like five people who like Zenos so keep that in mind.
Zenos is this powerful because he consumed what was left of the Mothercrystal. He's also now in a place powered purely by emotional thought, and he's basically harnessed homicidal psychopath. But he's still a mortal. Also I do think you're both tired from fighting Meteion.
Zenos is the rival to the WoL because he's your opposite. You fight for those you love. He fights for the sake of fighting. But you both enjoy the fight.
I think this was a satisfying end to Zenos. I think he's well and truly dead. He was satisfied that was the fight which killed him. He wanted to know if he could just kill you himself or if you're truly his better. After watching you kill Zodiark he knew his original plan of consuming a god would be pointless. He knew you're more than capable of killing gods.
I thought Zenos was gonna merge himself with Meteion and be a final fourth trial. It's what usually happens with Final Fantasy. Instead it ends with a fist fight where you beat him to death. I thought that was a nice twist on it.