There are two methods to netting this trophy, you can either do it online with another person ( a friend is always nice if possible) and the other is with 2 controllers offline.
The online approach - host or join a game already in progress. You are waiting for someone in the party to go into Fight For Your Life mode (where their health and shields are completely depleted but you have a limited amount of time to get a kill to revive yourself). A person that is down has a red circle indicator on their character model so it’s very clear when someone has gone down.
Approach the downed character and press and hold the Square bouton and after the meter onscreen fills you will revive the other player and this trophy should pop.
It is also possible to do this offline with 2 controllers, you as the main character on controller 1 can be signed into your account and your character. Then connect a second controller, sign in as a guest and just join as whatever Level 1 character you would like. Wander into an area of enemies with both characters and the Level 1 should be able to be downed quickly so you can revive them.