No matter how many billions, trillions, trillions, or billions of ATK your opponent has, there is some value in that alone, but basically, Kaiju and Thunderbolt are in the frame to consider this card
When your opponent activates a Spell/Trap card or effect (Quick Effect): You can negate the activation, and if you do, destroy that card
Remember the ideal time to cast a spell with flash is on the opponent's End Step, right before you untap for your turn
On the bright side, the copy will still be able to attack or tap if the original token had haste
Mutate doesnt stack the power and toughness of all mutated creatures
Ive personally only seen the Pikachu four distinct times, the Battle Frontier trainer card just once, and the Seviper card never yet - which is one of just two cards missing from my Seviper collection to fully complete it