This is one of the final ending achievements, so, spoiler alert.
At the end, you finally get to play your drinking games against Satan. The beer pong phase doesn't matter... win, lose, whatever. Eventually Satan gets fed up and transports himself, along with Milo and Lola, to some sort of extraplanar space to have the final showdown.
To get this achievement, you simply have to play the stacking game and win. This is a much more difficult version than you previously played, but Satan will give you a couple of tries... if you lose the first round, your other character will get to play, and when you lose that one, he'll offer to let you keep trying, because he wants you to win.
It seems to get easier after the first couple of rounds. I can confirm you can lose at least three times and still win on the fourth try, but I don't know if there's a limit to how many times you can retry. Just make sure that you keep choosing to try again... if you select the option "We'll just take the loss," you'll miss this achievement and instead get