Here's a written guide:
1. Go to Kastski, to the bottom right house, and talk to the guy inside to start the mission.
2. You'll need to go to Kanjanto Peak after the fourth boss, and go up. There's a hidden shop where you have to jump down, and you won't be able to see the house until you fall. Highly recommend the video as it's hard to describe, but it's not far as you ascend Kanjanto Peak.
3. Buy the elixir, and bring it back to the guy in Kastski.
4. Start the dream world and fight your way to the boss.
- The boss has two attacks. They will release a few creatures from their stomach that can be killed quick. They will also shoot three spread out fireballs from the eye on their face.
- When they shoot the fireballs, their eye will open up. You'll have a chance to jump up twice, and land 1-2 hits on the face of the monster.
5. Beat them, and you'll be teleported back to the house, giving you the achievement.