This Minku is in the cavern where you pick up the Misteria. From the entrance, hop onto the first moving platform, and go 90-degrees clockwise around the room - hop off onto the next platform you come to along the outer wall. The Minku is there.
If you need to, refer to BFM Part 9 for a reference on how/when to access the mine, and where, specifically, this cavern area is at: