Gloom taxes their white-heavy removal and threats, including Swords to Plowshares, Exalted Angel, and Meddling Mage, making it much harder for them to curve out efficiently. Simoon deals 1 damage to each creature your opponent controls, which cleanly kills their many 1-toughness creatures like Mother of Runes, Weathered Wayfarer, and Soltari Priest, though it will not touch sturdier bodies like Exalted Angel or Nova Cleric. Naturalize is cut down since their artifact and enchantment count is low outside of Seal of Cleansing, and Firestorm is awkward against a deck full of 1/1s and 1/2s that doesn't reward discarding your hand to kill small bodies when Simoon does it more efficiently. Keep your full creature suite and Zombie Infestation engine intact, since you want to race their aggressive white weenie plan rather than go fully defensive.