Afterparty

Drink 400 sips of alcohol/hellcohol. That equals 40 full drinks fully drunk.

This is quickest to do during the graduation party, right at the beginning, since you don't have to watch a long bartending animation. Just camp in front of the drink table, hold the right trigger down to chug your drinks and refill with the A button.

Your progress is not carried over between playthroughs. If you start a new game, you have to start over.

In addition to the fine guide above, there's also another great place to get this done: right at the end of the game.

You can keep replaying the stacking game there -- just make sure you select "Sure (try again)" when Satan asks -- and down as many drinks as you can while ensuring you're not stacking your cups. I got between 3-6% of that Achievement sorted with each stacking game. Just place your first cup to the far right, then hold down the left thumbstick to the left, keep the RT button held down and smash that A button as soon as your drink is done.

This Achievement is best paired with "Well At Least You Lost".

Here is my 100% Achievement guide & full walkthrough for Afterparty!
This achievement is for drinking 400 sips of hellcohol and where you could do it throughout the game, i found it best to do it at the beginning.

You find yourself in a college party, a bit of dialogue happens and you end up being asked if you want a drink..
You go to the bar, click "Sure" when the option comes up then get a drink using then hold down , rinse and repeat for 10 minutes until it unlocks.

Go to 3:50 in the video to see how to unlock it and where.

Hope it helps

Part 1

Part 2