Dimir Midrange is a mirror-style grind full of efficient interaction like Counterspell, Fatal Push, and Orcish Bowmasters, so this matchup is mostly about card quality rather than sideboard tech; most of our sideboard cards are either dead (White Orchid Phantom, Wrath of the Skies against a low creature count deck, Force of Negation which trades one-for-one against their own counters less efficiently than our maindeck) or narrow. Force of Despair is a cheap, free way to blow out a Psychic Frog or Orcish Bowmasters they just resolved, so bring in a small number for extra reach against their haste-less but tempo-sensitive creatures. Cut Persist since it is clunkier here than a cheap free answer, and keep the rest of the sixty largely intact since our Thoughtseize, Solitude, and countermagic-adjacent cards are already well positioned.