Did you try to level up various weapons for each of the characters or just stick with one weapon type throughout? There doesn't seem to be any inherent reason to use multiple weapon types other than variety, so I just picked one weapon for each character and stuck with it.
What about for magic? When I first played this game, I tried really hard to level up Maria's magic and get a Fire16, Blizzard16, and Thunder16, but it was slow going. I eventually gave up.
Funny that I really hated that aspect of the battle system yet something similar is how you level up weapons and magic skills in FFXI, a game I love. Of course, that game actually has levels and also involves the job system so that makes it a whole lot better!