This can be done in either online or offline co-op, and does not need to be done in one session. The revives will carry over across any co-op playing that you do. While playing co-op, when your partner loses all of his/her health, they will go down to the ground and will have a "bleed out" counter above their heads. This counter starts and 30 seconds and counts down to 0. If your partners body is getting hurt while downed, the counter will go faster. If the counter reaches 0, your partner dies and you both go back to the last checkpoint. For this trophy, what you need to do is wait until your partner is downed, run over to them and hold [Square Button] to revive them before the counter reaches 0. If your revive is successful, your partner will get back up with 50% health and one revive will be counted towards this trophy.
The fastest and easiest way to unlock this trophy is in offline co-op. Grab a second controller, if one is available to you, if not find a partner in the Co-op Boosting Thread. Log in your second player and load up the checkpoint: Haven, OK: Main Street Defense, on Superhuman. Once the level begins and the door opens, your team will leave the room you are in and begin to fight the Chimera. Now you should place your partner directly in the doorway, while you stay inside the safe room, just to the side of your partner.
If you are playing on Superhuman, within seconds, your partner will die. Now simply look at your partner, and hold [Square Button] to revive him. He will get up and have 50% health, then within seconds, he will die again, allowing you to revive him once more. Simply repeat this process until 20 revives has been reached. If playing online, then switch roles and let your partner revive you. After this has been done, the trophy will unlock.
Your progress towards this trophy can be tracked through the Extras menu, under the Rewards section.
I did not make this guide but obtained it from T_l_M_B_0 & AndrewAnonymous on PS3 trophy guide.