OMORI

When you first reach Otherworld by climbing the ladder east of the stump, go north/up to the next screen, leading to the Otherworld Campsite (Straight up from where Kel hangs Aubrey's BUTT CERTIFICATE). Follow the path leading up, and you should see Mari. Save your game, then continue going up, stopping when you climb the first small set of stairs. Immediately to your left you'll see a tree, with ??? hiding behind it. Speak to them and initiate the battle. Once you win the achievement should pop shortly after the dialogue. I don't know if this is missable, so I recommend doing it as soon as you arrive in Otherworld.