Mono-White Aggro has no blue or red cards, so Hydroblast and Envelop stay in the sideboard as dead cards. Cut Down is excellent here, killing Thraben Inspector, Novice Inspector, Raffine's Informant, Fortress Kin-Guard, and Lunarch Veteran, all well within its power+toughness 5 threshold, slowing their wide board development. Grim Bauble adds another cheap way to shrink or remove their one-toughness creatures before they connect or trigger synergies like Lunarch Veteran's lifegain. Reckoner's Bargain and the extra Blood Fountain are too slow against a low-curve aggro deck, so we trade them for proactive removal to protect our own tempo plan.