You will need to use the Cleric to heal people 10,000 times in order to unlock this achievement. The Sanctuary skill will not work for this achievement, you must use the Heal skill for the heal to count towards the achievement. This achievement will be very time consuming; the quickest way to get this is to spam the Heal skill as often as possible. Keep looking for team mates who get hit by enemy attacks, team mates who just finished building a tower, and team mates who are performing the Cheer and Sacred Song skills. These team mates will need to be healed and are good to spam the skill near, especially the ones involved in the Cheer and Sacred Song skills. It also helps to participate in these team skills because they will increase your HP and AP bars for a limited time; this will allow you to do more healing. Try to have equipment with buffs that will let you perform the Heal skill more often as well such as the "Conserve AP" buff.