When a member of your team is downed, but not killed, you will have the chance to revive them. When you spot a downed enemy, run over to them and revive them. Chances are an enemy will be nearby so keep an eye out when doing this as you will be vulnerable. This can be very easily farmed with a few friends. If you have friends on the other team, everyone can take turns downing players and letting the other player revive them.
Revive 10 Allies in Multiplayer