So I just had a weird experience in Pokemon Moon (want to finish Moon before starting Ultra Moon).
Found a place after the Electric Trial, just down the mountain with some grass.
Found a ditto that was kind of hard to catch along with some other Pokemon that I was able to get.
Then there was Beldum... fucking Beldum.
I don't think I have ever had as much trouble catching a non-legendary Pokemon as I did with this Beldum. And I don't mean finding it, I mean just what I said, 'catching' it.
First round: Quick Ball failed, using Snorlax's hold back and got it near enough dead, a few Ultras and NOPE, Take down killed it via recoil.
I then remembered I had Gengar and Take Down (and thus recoil wouldn't work) so I found another Beldum, hold back until it was almost dead (literally, about 1 - 10hp), switch to Gengar. Ultra Balls throw (quick ball didn't work again). and struggle recoil. :(
Found another one, Quick Ball failed so I went with the same strategy and again, 1 - 10hp. I even tried to get creative.
Long Story short, several Nestballs, a level ball, a pokeball and Gretball and 20+ ultraballs later (this 20 includes some of the Ultraballs from my previous attempts) and I finally caught it.
I have never had a non-legendary pokemon give me this much trouble catching it. finding them, sure but catching them... never.
But hey, at least I got it finally.