Question

Is nihilego the Pokemon hard to catch in Pokemon moon and to find

Comments

I had falsely assumed you were talking about Nihilego the body part so thank you for providing clarity

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I’m confused do you mean the hardest to find and catch?

Answer 1

It has a 90.80% catch rate at 1 HP and using a Beast Ball. With an Ultra Ball at 1 HP, the catch rate is 4.83% per ball, as per the catch rate calculator.

You can tell when a UB is available to catch in your current area, as the music will change to specific Ultra Beast score.
Using a Repel is a good way to speed things up, as UBs appear in standard wild Pokémon areas like long grass and this will cut out all the pesky Yungoos from your search.

this source has a walkthrough on how to find and catch each of the UB in the missions.

So, it can be hard to find and catch, but there’s things that can be done to make it easier.