Can I use Misty's Gyarados' "Splashing Panic" attack if I have less than 7 cards in my deck?
Yes, you would do as much as you can.
Splashing Panic






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Splashing PanicDiscard the top 7 cards of your deck, and this attack does 70 damage for each Misty's Pokémon that you discarded in this way.


WaterfallOnce it appears, it goes on a rampage. It remains enraged until it demolishes everything around it.
Can I use Misty's Gyarados' "Splashing Panic" attack if I have less than 7 cards in my deck?
Yes, you would do as much as you can.