She doesnt need to attack to get her main bonus: you can drop her in the first main phase and then send your modified creatures into the Red Zone right away for benefit, and while discarding a card to her ward is doable, most players will probably avoid 2-for-1ing themselves unless Arna becomes an emergency
The sneakier way is with effects that tax your opponents as they attack you, like Ghostly Prison
(There are shortages at the moment due to COVID-19, so dont buy an overpriced box.) Youve now got 24 packs to open, which means 12 complete decks at once, let alone all of the combinations possible by swapping those around
For a Base Set card, Gyarados is pretty powerful with 100 HP, Dragon Rage (which does 50 points of damage) and Bubble Beam, which inflicts 40 points of damage and has the chance of Paralyzing the opponents Pokemon too
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This commander, along with Korvold, Fae-Cursed King, can easily induce salt due to their ability to do everything, often leading to long, drawn-out turns without a guaranteed win