Boros Aggro is a mirror-style race with big dragon threats (Nova Hellkite, Magmatic Hellkite, Smaug the Magnificent) that our 2-damage Shocks can't profitably answer. Fiery Annihilation gives us better removal to keep pace with their bigger bodies while we otherwise keep our aggressive core intact to win the race. This is a minimal, surgical swap since our mainboard is already well-positioned for an aggro mirror and we don't want to dilute our proactive game plan.