If you just want a clean, straightforward one-turn kill, Chandra's Ignition is a pretty good option, able to smite every creature and player in such dramatic fashion that they probably won't really mind
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Pramikon, Sky Rampart Pramikon, Sky Rampart shakes things up a bit and breaks parity by limiting players to only attack the opponent to their left or right
[For Beginners] Raging Bolt ex Deck Guide (Gym Battle Winner Class) I'm Mizuki
Such an effect could normally mess up the enemys chance to play on curve
Drown in the Loch This one doesnt have the same high-profile results that the other cards at the top of the list do, but I believe thats just because it doesnt slot as cleanly into existing decks