The use of a Phoenix down or being on Blue Mage with the Angel Whisper or Transfusion spell equipped. Phoenix Downs are from chests inside of Palace of the Dead, Heaven on High or Eureka Orthos. The spells are both from items bought from a NPC after conditions are met. If you have Arcanist/Summoner or Red Mage unlocked and leveled high enough they both have a raise of their own as well.