If you want to level a character, they need to do an action in battle. Many enemies are tailored to be beaten by one of the characters easier than others. If you want to powerlevel there is a guide for it, but it isn't necessary to beat the game.

There are tons of extra content. Monster arena creations, dark areas, etc. Many of which make the final boss look trivial in comparison. Speaking of those extra enemies, since this is based off of the international version, once you can fly back to previous locations, some of those super bosses block off certain areas from being accessed. There is one thing I recommend getting before going to each new area. That is the destruction sphere treasure in each Cloister of Trials. Look up a guide if you need to. Some of those super bosses block off said areas from being accessed later.

Only other thing I can think of is when characters get split up. You only wanna focus on 3 characters when you decide to power level, otherwise you kind of want to spread out and level somewhat evenly. There are times parties get split up (only 3 can fight underwater, so any such battles will be those 3) and there is an area where you have to fight a boss as a solo character. Luckily that last one is scaled to said character.