I'm doing that right now, but right before I level up, I change all my characters to Blackbelts level 99 and equip all of them with:
Chakra Band
Black Belt Gi
Diamond Bracers
and the character with the least HP gets the Hellish Claws.
This way they get the maximum HP upgrade I can get. After they "Gain a Level" I change them back to whatever jobs they were leveling-up in, then do the grind again.
Until I get better gear, these are the gear that willl add to your Vitality stat the most.
Hope it helps.
