Warframe

Field Medic

This achievement is done by reviving 100 allies, does not have to be unique revives, you can revive 1 person 100 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 100.

Hi Everyone! I found a much easier way to complete this achievement, though it'll take a little preparation and a friend it make it work.
First, you need to unlock and build the Limbo warframe, you'll acquire all of his parts and blue print by completing the Limbo Theorem quest which is acquired after completing The Europa Junction on Jupiter. It also requires an Archwing to complete the quest.
Once you have Limbo built, find a friend that also has Limbo built, and head to the Plains of Eidolon on Earth.
Limbo has a special ability on his that allows him to jump inside another dimension called the Rift. While in the Rift, most enemies can't touch you, and while you can't hack consoles or pickup loot that's dropped, you can revive other players!
So once you're in the Plains, find a group of Grineer, the more the better, and have one Limbo stand there and take the beatings while the other goes into the Rift and stands by their friend. As the "real world" Limbo takes a beating and gets knocked down, the Rift Limbo can immediately revive them while taking no damage themselves! For a reference point, myself and another gamer got this done last night, ran about 100 revives in about 20-30 minutes.
Good luck Tenno!