Dimir Midrange is a blue mirror-style matchup full of blue spells and countermagic, so Mystical Dispute (discounted against their Counterspell, Force of Negation, Subtlety) upgrades your interaction. Day's Undoing is a wash-to-negative here since it refills a deck built to grind with card advantage already (Orcish Bowmasters, Psychic Frog), so it's cut. Trim Counterspell and Prismatic Ending slightly since this matchup rewards efficient, cheap interaction over sorcery-speed exile against their proactive threats.