Ghost Song

In the East of the map, below the Stoffbloom fast travel point, is a weapon called Flowersong. You can see it on the map here:
https://demajen.co.uk/#G

Equip this, and shoot flowers into the ground with in front of a spectator.

Several characters react to the flowers, but the only one I know for sure will pop the achievement is the secret shopkeeper, Mabec. [Comments also suggest that Gili, the girl in red and black, will work.]

Mabec is right in the middle of the map, just left of the elevator you ride early in the game. You need to come to the elevator and notice whether it's at the top or the bottom; then walk round to get in at the other end (no teleporting).

Once you're in the elevator shaft, jump up and slide down the left-hand side to find a secret passage. Follow that passage for the shop.

It is easiest to leave this till you have the spike shoes, though it's possible without.