This achievement is related to mission 11. Bastion's Fall
, not the level named Eden Prime. Slightly misleading achievement description.
Early on in this mission you will jump down from a high ledge in to an area with water running through an archway. Once you land on the floor, a bunch of corrupted enemies will spawn along with two uncorrupted angels. You must ensure they are not killed but killing all the enemies attacking them in this area. Naturally, I suggest completing this on an easier difficulty.
There are only a few enemies and you shouldn't have much trouble. I personally used Strife's chain lightning ammo to quickly damage the enemies. The achievement should unlock immediately after killing the last enemy, as the conversation with the angel starts.
Here's the clip I captured showing the area where the angels spawn: