I haven't been doing well at all. I've only ever caught two Legends, a single Articuno and a single Zapdos, and both times I was utterly riding the coat-tails of the other trainers present. I was just lucky they were strong enough to finish off the Legend before it could finish killing me. I'm hopeful for Entei, but I'm not hopeful for Mewtwo. That's just not a realistic goal for me yet.
He must have pretty high standards for IVs. I'm convinced at this point that captured Raid bosses follow the same rules as pokémon hatched from Eggs; always received at Level 20, and with no IVs lower than 10, when the maximum possible is 15.