For me it finally unlocked when I stayed in a room with Quagmire for 30 seconds again. I tried pretty much everything before that, but I remembered that I played that level in co-op, and when I did my actual playthrough I didn't go for that achievement again, because I already had it. Gladly I unlocked it then.
New XBL Profile since October 30th 2013.
Completed Retail Games: 16
Completed Arcade Games: 18