Instructions
Find a monster encounter. As in many role-playing games, the more powerful the monster is, the more experience it will be worth for increasing your weapons and characters more quickly.
Fight the monster with a more powerful character. Toran or Goro might have stronger weapons that deal more damage than Ruby's bracelets, for instance, especially while they're weaker in the earlier levels.
Switch your fighting character to Ruby just as the monster is dying. This way Ruby and her bracelets get the necessary xp for participating, and you can level her equipment up much more quickly. Check the inventory screen under "Weapons" to see when Ruby's bracelets are ready to evolve and change.