When one of your CPU-controlled allies dies (once they are completely out of life points), you will see a 60 second countdown at the top of the screen. Within this 60 seconds, you need to navigate the pointer over their character portrait in the status bar at top of the screen and 'click' it with the R3 button (or L1) to revive them (this costs 50*lvl of companion in gold for a first revival). Doing this for the first time will earn you this trophy.
You are almost certain to have opportunity to get this trophy over the course of this game - it is challenging enough and the AI is unskilled enough that some amount of death is inevitable. However if for some reason you want to get it the absolute soonest, the easiest place early on to ensure death of a companion is on the area with the spike traps towards the end of the first dungeon level (Ancient Temple Ruins). Simply stand to the edge of the traps and your ally will attempt to stand next to you and get hit repeatedly by the spike traps until dead (this may take several minutes).