In order to get the trophy that's called "Friendship Rules", you need to have revived someone from "Fight for Your Life!" that is on your friends list.
To begin, make sure you have a friend on your PSN Friend list that owns the game as well as you owning a copy of the game too. From there, either start a co-op game or join him or her in a game of their own or vice-versa.
At some point in the game, it doesn't matter what enemy you face, if they take out your shield energy, along with your life energy, you will enter what's called "Fight For Your Life!", a second chance at killing someone and gaining a second wind. If they cannot get out of that jam, then it's up to you to revive them fully.
When a friend of yours gets into that stage, make sure you are standing or crouching near them and hold down
. A green lifebar will appear and it will take at least ten-fifteen seconds before you fully revive them. Once you have revived them from "Fight For Your Life!", they should get back up and the trophy should unlock right then and there!
According to TheWolf, it's also possible to get this trophy on your own on split-screen with two PSN accounts activated on the same console, as long as you have two controllers.
Have fun and good luck!