Use Revival Beads
Use Chakra Drop or Chakra Pot if the demon is out of MP and cannot act
Visit a Leyline Fount and restore the party if the demon is in the active stock
Use healing skills that revive allies, such as Recarm-type abilities
Use items or skills that target fallen allies during battle
Bring the demon back through the Demon Compendium if it was dismissed or lost outside battle
Re-summon the demon from the Compendium at a Leyline Fount
If the demon is in the reserve, swap it into the active party after reviving it
If the whole party is wiped out, reload from the last save or autosave