Affinity's entire gameplan revolves around cheap artifacts, Urza's Saga, and Mox Opal enabling explosive draws, making Meltdown our single best sideboard card. At X=0 it destroys Mox Opal, Mishra's Bauble, Tormod's Crypt, Welding Jar, Claws of Gix, uncharged Engineered Explosives, and every Urza's Saga Construct token; at X=1 it also picks off Pithing Needle, Shadowspear, and Skateboard. It does not hit Kappa Cannoneer or Pinnacle Emissary, so Counterspell and Unholy Heat remain our main answers to those threats. Both copies of Meltdown come in for the two least impactful Spell Snares, since their curve skews toward 0 and 1-mana artifacts that Spell Snare can't touch (and Pinnacle Emissary/Kappa Cannoneer cost more than 2).