Stiflenought wins by using Stifle to counter the Phyrexian Dreadnought's triggered ability, then attacking with a 12/12 trampler. Meddling Mage is your best answer — naming Stifle stops their entire combo cold, and naming Phyrexian Dreadnought as a backup shuts off the threat itself. Phyrexian Furnace and Powder Keg are largely dead cards in this matchup since they neither interact with the combo nor affect the blue battle meaningfully, and Humility is too slow and doesn't answer a resolved Dreadnought before they attack. Keep Counterspell, Mana Leak, and Prohibit live to fight over Stifle and Dreadnought on the stack, and use Swords to Plowshares as a clean answer if a Dreadnought resolves.