In order to unlock this Trophy you must carry a tome to the end of the level. The tome takes up your healing item slot, which can make the missions a bit more tricky. As a result I recommend doing this one Easy. The video below by Elgato Gaming and posted by 360GameTV (I take no credit for this video) does an excellent job of showing how to find all three of the tomes in the prologue mission, The Horn of Magnus, (but you only need one tome for this Trophy)...enjoy!