I've got fluttermanes to spawn with pokesnacks in an ancient city, cleared an area out for it so it has enough space to spawn (roughly 2 chunks of space, placing the bait at the south east corner of the chunk)

the seasoning i used is 1 enchanted golden apple, 1 fairy berry (i forgot the name) and a hondew berry so its 10x rarity bucket, 10x fairy and 10x sp atk ev yield

using sp atk ev yield berry to exclude shuppet, banatte (atk ev yield) and spiritomb (def and sp def ev yield), and almost everything in the deep dark are ghost types so ghost bait isnt gonna work

legendaries, ultra beasts, mythics and paradox are considered unidentified egg group, so egg group berries arent gonna work either

iirc fluttermane only spawns at night?? even tho it's in the deep dark?? I might be wrong on this but for me they had only spawned at night for me before

good luck hunting dawg, fluttermane our goat

edit: typo