As long as Tentacruel has any React Energy cards attached to it, prevent all effects, including damage, done to any of your Tentacruel in play by attacks from your opponent's Pokémon-ex.

Water Arrow

Choose 1 of your opponent's Pokémon. This attack does 20 damage to that Pokémon. (Don't apply Weakness and Resistance for Benched Pokémon.)

Fury Strikes

20×

Flip 3 coins. This attack does 20 damage times the number of heads.