Containment Priest is strong here, but only against part of their plan. It exiles Atraxa or Acererak if they are put onto the battlefield with Show and Tell, since a nontoken creature that wasn't cast gets exiled instead. It does not stop Omniscience, which is an enchantment and still enters off Show and Tell. It also does nothing against creatures they actually cast, such as Acererak through Aluren or Atraxa hard-cast. Force of Negation lets you counter Show and Tell, Aluren, Stock Up, or Omniscience for free on their turn, which covers the gaps Priest leaves. This matters because Veil of Summer makes them hard to interact with reactively once they're ready to go off. Stifle is weak here since their key enablers (Aluren, Show and Tell) are static effects or spells rather than abilities to Stifle, and Phyrexian Dreadnought is risky to commit mana and creatures into against a deck that can go over the top with Atraxa or Omniscience. Prioritize keeping Force of Will and your cantrips to find these answers quickly.