Afterparty

This achievement is missable and occurs towards the end of the game, so spoilers ahead if you haven't played through the story yet.

After the fight where Milo and Lola get split up, whichever character you are goes back to 1st and Izzard to re-unite with their friend. Upon arrival, Sam is at the bar, and points out that your friend is over at the beer pong table. No, not your human friend, your other friend, Wormhorn.

Go speak to her before going upstairs -- if you go upstairs first, Wormhorn will be gone and you'll miss your shot. She's feeling depressed about the fact that her existence will be over soon, so be nice and offer to play a game of beer pong with her. She'll tell you to go get your friend first, so go upstairs to find out what your counterpart has been up to, then come back downstairs and sit at the bar to reconcile with them.

After you've hugged or high-fived it out, instead of just leaving the bar, go over and have your beer pong game with Wormhorn. Again, try to say nice things... I don't know if that makes a difference at this point, but it can't hurt. The achievement popped for me as soon as the beer pong game was over.

This is Missable.

After Milo and Lola fight, Sam will text you to say "Go to 1st and Izzard" and the Schoolyard Strangler bar.

Do that, and as you arrive, do NOT go directly up the stairs! If you go straight upstairs.. Wormhorn will be gone and you miss your opportunity!

So once you enter the bar, go straight to the right, have a chat, and she will want you to play beer pong.
Then you can go upstairs, make up with Lola, then before you leave, go right again and have that beer pong game with Wormhorn and job done!

Go to 1:31:00 on my Part 2 video to see how its done and where it unlocks.

Part 1

Part 2