Hey Community,
Glory Kills can be performed after getting an enemy low on health (blinking blue if you are too far away to perform a glory kill by pressing the melee key and orange when you can perform a glory kill by pressing the melee button).
You can kill every enemy in the game with Glory Kills.
If you use Glory Kills occasionally, this achievement will pop-up early in the game. There are much more than 33 unique Glory Kills. Each type of enemy has a specific amount of locations to perform different form of Glory Kills. Just hit the melee button from different, for instance from the front, behind, sides, from below and above.