Agent of Treachery is great when their planeswalker is about to ult. Also, while the new Vorinclex, Monstrous Raider is good with planeswalkers, his second ability is good against them as well. Finally, as Cranky said earlier, planeswalkers fold to aggression.
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[Modern] Allies
I had a super-tuned $10k planeswalker deck for a long time and while it was fun, it always fell apart when aggression happened.