Spoilers ahead, I guess. Nothing story related, just about espers.

I beat Zalera, Adremmelech, and Cuchulam at about level 35. I proceeded to whoop up on Zeromus at about 47 and Exodus not to far later.
Am now level 67 (Prime number =D) and haven't attempted Chaos because I don't have the medallions, or Ultima--just because.