Think about it this way - you can only kill the eye with a decoy potion, which means that you have to be standing near those two decoy potions at the top of the room the move before you place the decoy and kill it. The eye also has to be standing within one square of an empty decoy-placeable square when you do this.
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×Therefore, you have to kill the eye with a decoy placed on the northern target square - if you already placed one there, then you can't solve the room.
So, with that little hint in mind, and re-examining everything in the room, you might be able to come up with an alternative method that will work. Feel free to ask for more explicit hints.
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Quick links to my stuff (in case you forgot where it was):
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