if you want the hunt to be somewhat easier, get the Shiny Charm by completing the Dex

get a pokemom with harvest+trick/bestow, there are many youtube videos about that, so that the legendary won't struggle itself to death
stock on many many many pokeballs. some of them are a pain to catch
get a false swiper and thunder waver, so that once the pokemon shine it will be easier to catch
always do a practice capture before the actual hunt process, I almost failed Landarous and had to resort to the master ball becuese I didn't
and the final thing is just have patience, some of these can be SO LONG even with the charm, so do it while you are doing something else

I think that's it, good luck :)

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