How can i get crystalmouth fast?

Neo Monsters
1

I currently need a crystalmouth to evolve my legendary but the super ultra-evolve ingredient missions are locked for a week.
When i checked it said that i can find it in golden halls but I couldn’t find golden halls mission

2

He’s very rare

3

Golden halls is the goldmouth level. Every time you play it, there’s a small chance of encountering a crystalmouth. Doing this is slow, inefficient and unreliable, but still the quickest way to get a crystalmouth on demand as far as I’m aware

4

Thanks a lot