I did exactly the same, actually. It was the last Pokémon I had to catch, and I still had my Master Ball, so I thought "why not?" :DI caught my Landorus with the Master Ball :)
Yes, i used it on Landorus because it is the last, anyway, im lucky and i catch a lot of pokemon without it, give a tip? buy a lot of quick balls, its the best, not really, just to catch fasterI did exactly the same, actually. It was the last Pokémon I had to catch, and I still had my Master Ball, so I thought "why not?" :D