Mono Red Rally is an all-red Burning-Tree Emissary / Rally at the Hornburg aggro-combo deck, so Blue Elemental Blast is live against nearly everything they cast, including Rally at the Hornburg itself, Lightning Bolt, Galvanic Blast, Chain Lightning, and Goblin Bushwhacker. Countering Rally at the Hornburg directly deflates their alpha-strike plan before it generates lethal damage. Last Breath stays out since their cheap creatures (Burning-Tree Emissary, Clockwork Percussionist) get their value from an enters trigger or haste that has already happened by the time you exile them. Trim Meeting of Minds, which is too slow against this fast combo-aggro clock, and Negate, which overlaps with Blue Elemental Blast on the same burn spells at a higher cost, relying on your 0/3 and 0/4 walls to buy time.