Cost-Efficient Card
Starmie EX is one of the most cost-efficient cards that can deal 90 damage with 2 Water Energies. It has the advantage of defeating a Pokémon within 2 turns and requires 0 cost to retreat. This will let you retreat easily when necessary.
Support Damage with Greninja
There are situations where Hydro Splash will be against a Stage 1 Evolution or higher. In this case, use Greninja's ability, Water Shuriken, to support the damage. This will let you deal 110 damage and defeat the Pokémon in one hit.