Healer
This achievement is done by reviving 10 allies, does not have to be unique revives, you can revive 1 person 10 times and obtain this achievement. To revive an ally while they are downed, run over their body and hold the B button, be careful if they are surrounded by enemies, unless you have an ability to clear a group or blind ability you will most likely die.
Something worth noting is that if you assist in a revive (multiple people can revive 1 person) that does go toward this achievement, so even if someone is reviving a person, join in quickly to get another 1 towards the 10.