Ive used one in masters a fair bit, does well in my experience, doesnt fear legacy dragonite as much as other dragon move carriers.
All its moves are good in my oppinion, HP's good against groudon/metagross/tyranitar, crunch for latios/lugia, outrage for latios, dragonite. But I'd recommend Hydropump and crunch.
Partners, i'd say mamoswine, much, much better as dismantling the dragons in my oppinion, defensively good against melmetal, and has better moves than groudon (though i'd recommend giving it bulldoze as a second move against meltetal and the occasional raikou). After that, Either metagross or Tyanitar (B/C + stone edge if possible), gyarados matches FB and SB mewtwo well (who's a bit rare and somewhat predictable as a late game sweeper who needs to be partnered with shield breakers early on to be threatening), walls groudon, and machamp and fire types are things you dont really need to worry about in masters
