Octillery.

Reveal the top 5 cards of your deck. Flip a coin for each Energy card you find there. This attack does 50 damage times the number of heads. Shuffle the revealed cards back into your deck.

Any time the Defending Pokémon tries to attack, your opponent flips a coin. If tails, that attack does nothing. (If the Defending Pokémon is no longer your opponent's Active Pokémon, this effect ends.)