Mono Red Rally chains Burning-Tree Emissary to generate extra mana and flood the board with cheap creatures, then closes with Rally at the Hornburg to flood the board with hasty tokens, so you need to survive the alpha strike and stabilize with sturdier bodies. Breath Weapon is excellent here, clearing their swarm of small creatures right when they try to go wide before a Rally. Guardian of the Guildpact is nearly untouchable in this matchup — protection from monocolored means it shrugs off their red removal spells and can't be profitably blocked or outrun by their mono-red creatures. Cut the slow card-selection pieces that don't help you race or block, and hold Journey to Nowhere for their most dangerous creature threats rather than tapping out early.