Final Fantasy II is a game that eschews the normal gain experience to go up in levels system. Instead you have levels for everything you can possibly do and you get better at something by doing it. Want to get better with a sword? Attack with a sword. Want to get better with Ultima? Cast Ultima. Want to get more maximum health? Take a bunch of damage.
Unfortunately the system is wide open to abuse. Want to get better with swords, and with shields, and get more strength, and more vitality, and more maximum health? Try hitting yourself in the head with a sword over and over and over again. The poor imp you're engaged with won't know what's going on but you don't care!
I looked around a bit and couldn't find any challenges to do in this game. In fact, it just seems plain hard and no one really seems to like it very much. I've only beaten it once so just playing through normally could be the right plan... But how to define normally? Normally for me where I'll hit myself with a sword for 15 hours and then cakewalk through the game or normally in the sense that my characters will only take actions I normally would in a different game. No hitting yourself with a sword. No cast life on people who aren't dead. Etc. I worry this will make the game practically impossible though since I simply won't get to level up spells like life to the point where they're useful.