Level 45. Go and obtain (SPOILER)Zalera and Adremmelech. Then level up to 56, go and get (SPOILER)Exodus and Cuchulainn. And after a little bit of levelling, kill (SPOILER)Chaos and Zeromus. Lastly at level 75, (SPOILER)go beat Ultima and Zodiac.

Just remember that some Esper seal one of your command, (SPOILER)Exodus seal Item Command, Zeromus seal Magicks Command [You can still use Technicks], Chaos seal Fight Command, Ultima and Zodiac can seal any of your command when she/he pleases, also set something tricky, (SPOILER)Cuchulainn set a HP Draining Field, your HP will be constantly drained like Sap status do, Quasimodo Boots/or any Sap-protecting things won't help you and you must beat Zalera within 5 minutes., they also have minions (SPOILER)Adremmelech, Zalera and Zeromus have zombies (They cast Dark-elemental Magicks a lot, equip Black Mask on everyone to absorb it) coming, Cuchulainn have Foobars [A normal enemy in Nabreus Deadlands, cast Mythril Bubbles a lot, Mythril Bubbles give more status than Cloying Breath, Bad Breath and Putrid Breath, becareful] coming, Chaos have 4 Chaos Judas [Each have about 4000 HP] coming., lastly set a good defense on themselves, (SPOILER)Adremmelech and Chaos are flying, Exodus and Zodiac sometimes set a Perfect Defense which makes them immune to Physical Damage and Magickal Damage, even Quickenings won't do damage, I dunno if there's another Esper that knows Perfect Defense..

This is about STORY ESPERSBelias is fine and easy, leave him alone. Mateus comes with Ice Azer which have about 2000 HP, wipe them out with Thundara or Quickenings. Shumihaza knows nothing special, aside from her Silencega, Flare and Shock. Hashmalim is a joke when you cast Float (Better if you have a permanent Float [An accessory]), Ruxxor and Quakeja will miss, you just have to becareful of his physical damage. Famfrit is with Doctor Cid, like when you fight him before, you can't damage him until you beat Famfrit, heal when Famfrit cast Waterja. Dispel Famfrit to keep Doctor Cid busy rebuffing the dispelled Famfrit. Doctor Cid suddenly do more damage after Famfrit is gone.
They're just tricky.