15. DC Universe Online - "Not Even A Scratch"
While most of the trophies in DC Universe Online provided basic challenges like killing X amount of enemies or completing a mission with only taking so much damage, you often couldn't unlock any of them unless you arrived at their respective missions completely over-levelled. The same was true for the "Not Even a Scratch" trophy, which has you defeating Ra's Al Ghul without allowing Felix Faust to take any damage.
You could replay the mission over and over until you somehow fought your way through the battle with sheer luck, or you could keep a save file from before the mission and go back to grind up your level, but even these tactics only slightly decreased the chance of Faust taking a hit regardless.
Trophies where you have to rely on AI are always frustrating and they feel cheaper still, because no matter how skilful you are, gaming AI always has a knack for running straight into oncoming fire and getting themselves mutilated.