Achievo requirement is to revive a cumulative total of 50 coop partners. They can be the same player 50 times, or 50 different people, in horde or campaign, and does not need to be done in one session. They do NOT need to be on your Friend list.
To revive someone: press and hold
when close to a downed player.
Because of “auto revive” and self-resuscitating perks in the game, this might take a long time as you might not get much opportunity in regular play to revive anyone.
However, a quick way to boost it is to set the game to HARD, because this enables friendly fire.
Then just load up any level & chapter (beginnings of chapters are largely void of zombies) and , with a cooperative coop partner just shoot and revive each other until you have reached 50 revives. Will take 20 minutes or less.
Achievo will pop as soo as you have attained a cumulative total of 50 partner revives.