Good suggestions. I was planning for only one miniboss in the current region, but it'd be possible to encounter him as a notable enemy, similar to Techies. It would be almost ideal for Geno to have such an encounter to keep him occupied until the rest of the team arrives, but unfortunately Geno doesn't have the ability or know-how to free spirits from Galeem's influence without killing them first.

Since I imagine it'll be too long before someone with the know-how reaches Peach's Castle, it makes the most sense to encounter him as a notable enemy in Ancient Gardens (the very next area), perhaps just wrapping up playtime with some Rabbids as a main character wanders off and discovers him.

Would any non-Geno individual or pair from the Central Route party care to volunteer for such a role in the near-future? Since it would be its own encounter, there would be an associated reward, and reward increases with fewer party members and higher difficulty.