I received this without actually reviving a teammate. Somebody else in my team did.
However, just in case it doesn't bug for you, when somebody has their health depleted, they will fall to the ground and can't move, although they can continue to shoot.
You will know they have fallen, because their name will appear in red on your HUD.
Run over to them, and hold
until they are revived.
Some achievements won't pop until the end of the round, although this one seems to pop right away. Although, other people report they had to go back into the lobby for this to pop.