Here's a neat trick:
Some characters aren't good at leveling skills. That would be those
with letters like C, D, E and F for skill aptitudes.

But if you want to use these characters anywhere, there is a simple
way to level up some of their skills quickly. Just summon the phantom
you want to level in battle, and summon another phantom with an S or A
in the skill type you want to level. Then have the high aptitude
character pick up the low aptitude character, and have them attack
with the skill you want to level! For this to work, the skill must be
part of the phantom, not a weapon.