Dimir Midrange has a deep suite of counterspells (Counterspell, Force of Negation, Spell Snare) and removal (Fatal Push, Thoughtseize) that outgrinds a deck that runs out of gas. Thoughtseize fights on the same axis, letting you strip a counterspell or removal spell before you commit your own threats. Dreadhorde Arcanist generates card advantage by rebuying Lightning Bolt or Lava Dart from the graveyard, which is critical in a matchup where every card matters. Mutagenic Growth and Violent Urge are cut because they're most useful in combat-heavy races, and this matchup is decided by resource attrition instead.