Srsly. I've gone through at least that many when farming Fei'yin. Going to have to say SS or it didn't happen when it comes to these dropping chest keys.
I've killed about 200 Underworld (yes Underworld, not Undead) Bats in the past month and haven't gotten a single key. So if anyone's looking to get a Fei'Yin Chest Key... I'd try one of the other key-dropping mobs first XD