Mono Red Rally floods the board with small tokens and cheap creatures like Burning-Tree Emissary and the Human Soldier tokens from Rally at the Hornburg before pumping them for a lethal swing, so Arms of Hadar's -2/-2 to all their creatures is a great one-sided answer that kills tokens and most of their small creatures at once. Extra Thraben Charm copies scale with your own board and reliably kill Clockwork Percussionist, Goblin Tomb Raider, and Voldaren Epicure before they generate value or attack. Cut the slower value cards like Grim Bauble, Eviscerator's Insight, and Omen of the Dead since this matchup is decided by whether you can wipe their board before Rally at the Hornburg goes lethal, not by grinding card advantage.