You must revive all 3 other players in the same level in a 4 player game.
Can be done with 3 split screen contollers on Xbox, on Playstation requires 2 players with splitscreen or 4 players.
When they are knocked out, walk over and hit
over your fallen comrade. Hit the heart in the middle for most health or mash
for a quick revive. Do this to the 3 other castle crashers in your game and Medic! should unlock