Where to find Slater's recipe
The achievement becomes much easier once you have already reached Slater's room later in the story. In that scene, inspect the recipe board or poster in his room to see the exact mix for his secret slushy.
The important part is that the game shows you the answer after the first blind slushy moment, which is why this achievement is easy to miss on a natural first run.
How to unlock Grunge Metal Alchemist
- Progress until you have seen Slater's room and checked the secret slushy recipe.
- Open Chapter Select from the main menu.
- Replay the gas station slushy sequence.
- Make one slushy using the four confirmed flavors: Blue Crush, Bubblegum, Orange, and Lime.
- The achievement should unlock as soon as the correct drink is accepted.
If the unlock does not pop
Double-check that you used all four listed flavors in the same replay and that you were in the actual slushy chapter rather than just revisiting Slater's room. If you experimented during your initial story run, that does not hurt anything. Just replay the sequence and remake the verified drink.