Mono-Red Aggro is a fast creature deck with cheap threats and burn to close out the game, so a sweeper like Day of Black Sun is excellent, wiping their early board of Hired Claw, Magda, and Taurean Mauler while your mana lets you scale X. Countermagic like We Say Thee Nay! and Spell Pierce is weak here since they're not relying on a key noncreature spell to win, instead applying constant pressure with creatures and direct burn like Burst Lightning and Lightning Strike. Flashfreeze provides efficient interaction by cleanly countering any of their red threats or burn spells while you stabilize. Keep your dedicated removal (Bitter Triumph, Requiting Hex) fully intact since this matchup is decided by surviving the early turns and taking over with card advantage.