Affinity is almost entirely artifacts, so Meltdown is devastating, sweeping away Mox Opal, Welding Jar, Engineered Explosives, and other cheap permanents for a small X. Lightning Bolt struggles to kill their bigger threats like Kappa Cannoneer once it has counters, making it a worse removal spell than Meltdown in this specific matchup. Obsidian Charmaw gives us a flying 4/4 whose enter trigger destroys a nonbasic land such as Urza's Saga, and it can cost less when their lands can produce colorless mana, like Urza's Saga or Shivan Reef. Street Wraith's swampwalk is dead since Affinity plays no Swamps, so it's fine as a cut.