Mystical Dispute is cheap against their blue spells (such as Neoform, Pact of Negation, and Mystical Dispute itself if they board it in), giving you efficient counter-magic to stop their combo pieces on the stack. Celestial Purge does not hit their mono-green cards, so it is a marginal include primarily useful against their black or red permanents (such as Griselbrand, Xenagos, Atraxa, or Ureni); still a fine catch-all instant. Orim's Chant is less effective against a combo deck that can assemble Eldritch Evolution/Neoform chains in one turn, since they can respond with a free spell via Pact of Negation or resolve Allosaurus Rider before Chant takes effect. Keep Counterspell, Spell Snare, and Teferi, Time Raveler as your primary interaction to disrupt their one-turn kill before it resolves.