This is essentially a mirror match, so the deck that generates more value off each card tends to win. Dreadhorde Arcanist lets you re-cast a cheap Lightning Bolt or Lava Dart from the graveyard on attack, generating extra reach and card advantage the opposing list can't match. Thoughtseize taken proactively can strip their best spell (Expressive Iteration or a removal spell) before it can swing the race in their favor. Mutagenic Growth and Violent Urge are marginal combat tricks in a matchup decided more by resource efficiency than combat math, so they're cut for the extra bodies and disruption.