To start off, you have to have Playstation Plus in order to get this trophy. There is no way around this fact.
This trophy is given to you once you revive (or help revive) a fallen brother. This is done by walking up to the assassin where they are lying on the ground and, once you see the
Revive option, holding
until they can get up (you fill in the circle). You can get this trophy by helping out reviving (even if someone else is reviving a downed player, you can start holding
and will help them raise the player up - though this is harder to do as the game will just not do it sometimes - I did do it once though, so it should be possible).
The easiest way to do this is to have a friend join a co-op mission with you and have them get knocked out. Then you can kill off the enemies around and revive them (Just make sure they don't hold
while they are down or they will desynchronize and you won't get the trophy). If you don't have a friend playing with you, you will just have to wait until an assassin get's knocked out (you'll know they are knocked out as their Assassin icon will change to a skull icon). Of course you could always just ask the people you are playing with if they would die for you to get the trophy...
Also, just to note, unless you have a friend playing with you, when you start a co-op mission, you must choose Play in Public (the
option). Playing in Private without a friend makes you play the mission alone and you can't get this trophy by yourself.