X-Meteo, if you have all the best weapons and armour (and consequently, spells and abilties) from finding all the chocograph chests, I doubt the final boss would have been too much trouble at level 40, although it may have been tricky. I found this to be one of the FF games that didn't require any grinding to complete.

I remember those diehards who used to go for level 1 challenges. Dunno if any of them ever succeeded.
Yes, level 1 is not that hard to pull off in this game because equipment is more beneficial than level. If you go back and do Chocobo H&C all the way through for some ultimate weapons and armor, level 40 isn't too low for taking on the final boss. I'd recommend trying to get the team to at least 45, though. 40 is fine for Kuja but the other guy is a bit much.