658 Greninja

Bubble falls slightly behind Feint Attack in damage output, but offers more energy gains to make up for it. The latter sports average damage output and energy generation, with Dark-type coverage. 

Night Slash costs the least amount of energy, benefits from STAB, can be used frequently, and has a small chance to proc its self-buff. Surf is a solid move overall, also gets STAB, and doesn't require a lot of energy to use. Aerial Ace is quite underwhelming; its coverage against Greninja's weaknesses is likely not enough given its poor damage and greater energy cost. Hydro Pump demands too much energy to use. 

Greninja shares the same Water/Dark typing as Sharpedo, with similarly high offensive stats and fragile bulk. Fortunately for Greninja its moveset is a lot faster and more efficient, with greater energy generation and more frequent shield pressure. Water/Dark typing is not particularly synergistic as they don't cover each other's weaknesses, but gives Greninja advantages and disadvantages that other Pokemon don't have. The main drawback with Greninja is its poor bulk, and to a lesser extent, its relatively weak moveset, with losses against various neutral matchups like Skarmory. Overall the fragility of this Pokemon might discourage its use, given the numerous other existing options available. 

Greninja is generally not recommended for this league, with various Grass, Fighting, and Fairy-types in the meta. Feint Attack is likely the more appropriate choice given the ubiquitous use of Giratina and bulky Psychic-types, though this difference isn't too notable in Premier Cup. 

Greninja desperately needs stats, especially bulk, if it's to compete in Master League. Feint Attack helps win the Metagross matchup, but the rest of the meta will be a major challenge.