This is Missable.
Basically directly after getting the "She likes you enough, i guess" Achievement where you make friends with Sam. You will be on your final taxi trip to satans.
You will then be asked who you want to accompany you with during the beer pong phase of the "Battle".
You get a choice of one demon (Or person) and another option to Move On.. Always make sure to click Move on until the options exhaust themself and you have a random (For me, it was Lynda) and the drunk idiot demon.
Go to my Part 2 video at 1:51:20 to see how it unlocks.
Part 1
Part 2