WU is a heavily blue deck, so Pyroblast is excellent at countering their Brainstorm, Stifle, Force of Will, or Consign to Memory, or destroying a problematic Tamiyo or Quantum Riddler. Meltdown can handle nothing relevant here since they run few cheap artifacts, but it's still a cheap sorcery-speed answer if needed against incidental permanents; consider it a flex include. Unholy Heat struggles to profitably kill their bigger threats like Quantum Riddler or flipped Tamiyo, so it's trimmed. Boomerang Basics is replaced by more proactive, cheaper interaction since this matchup is about racing through their tempo plays.