Well after you unlock the doors to the chamber, you either fire
ultra balls at it or if u can find a luxury ball use that. it is
easier with a luxury ball. I was really lucky. I caught it with my
one and only thrown ultra ball. dont get your hopes up that you
will because it is insanely hard. bring a bunch of ultra balls. i
brought 91 and left the cave, with regice, and 90 ultra
Go in the island cave then use Fly in the middle.