Suicune is okay in the Master League with Ice Fang, Bubble Beam, and Ice Beam/Hydro Pump. It also works in Great League but 100 IV Pokémon are better in the Master League. The downside to Shadow Pokémon is that they take more damage (But they deal more Damage). Suicune is supposed to lower the Opponent's Attack with Bubble Beam and take hits, while it might still be able to do so, its will go down faster than a Normal Suicune.
But if you want to use it, its better to use it for the Master League.