it might be a trap, kirby could've ate everyone XD
and that just kind of comes with the territory for nippon ichi post game fights, everything is incredibly overpowered and eventually the player's characters get there as well.
main game stuff is pretty tame by comparison, strongest characters might be getting in the 10k range stat wise maybe, possibly just breaking 100k for 4. In my current post game, think my strongest character has at least 20 mil atk, going to be much more within a day or two as well.
Far as final fantasy super bosses go, think I beat emerald in 7, ultima in 8 for the eden GF, ultima in tactics, though I don't recall if I learned zodiac from him or not, forget which ones I did in X, know I did at least one weapon, did a good chunk of the monster arena ones, and don't think I've fought one in any of the games beyond that XD.
seen videos of the omega fight in 8, seems like the main ways to go are lion heart, irvine's ammo limit, or being really good at zell's limit.