While the trophy description is relatively self-explanatory, there are two key places where it is easy to make a mistake that I wanted to point out.
1) Recruiting the ogre: You must recruit him while he is still under attack. If he kills all of his attackers, this can make it not count for the trophy.
2) Freeing the ogre: In the Palace District, you will lose the ogre and need to reenthrall him. Note that a second ogre will spawn very near by. Make sure you enthrall the one closest to the gate. If you got the right one, he should still have the same approval level he did when he left the party.
If you make a mistake, just reloading may not be enough; on Xbox you needed to restart the whole module. I don't know whether or not this applies to PS3, but the DLC is so short that you may want to do that just to be safe. If you're playing on Casual, you shouldn't have anything to worry about (even during the fights with Sten or at the end, the two toughest parts of the module).