On level 11 head up to where you meet Deep Coat who is in the toilets on the left. But instead go to the toilets on the right and you will see several toilet stalls and a lot of enemies. Take them out and in the middle you will see three toilets together and a picture on the wall with three dots. Flush the toilets in order of the smallest circle first up to the biggest.
I don’t think the order changes but for me it was,
Middle,
Left,
Right.
Once done a secret will rise and achievement will pop.
(Also I’m aware I used the word toilet a lot!)