To hurt the eye boss you have to have the pandora box in your inventory.
To get the pandora box you have to fight the optional boss from previous level.
If you are reading this you probably already in the room with an eye and overwritten your solitary allowed save file.
Now you can start over with this knowledge.
Or you can pick the options "watch" the walkthrough of this game inside this game, and when that player enters the eye room you can press the play from here button and assume direct control.
If that's not enough for you and you want the achievement, then assume direct control, save, quit, load and continue playing. Enjoy that until that's patched.