“Mono-Blue Stiflenought wins by cheating Phyrexian Dreadnought into play as a 12/12 trampler on turn one by using Stifle or Vision Charm to counter its 'sacrifice creatures with total power 12 or greater' triggered ability. The deck protects this threat and its combo pieces with a dense suite of free and cheap counterspells (Counterspell, Foil, Daze, Thwart, Misdirection), while cantrips and card draw (Opt, Portent, Gush, Impulse) provide the consistency needed to assemble the two-card combo quickly. The strategy is essentially a tempo-combo hybrid: land a 12/12 on turn one and then protect it long enough to kill the opponent in two attack steps.”
Missing sideboard cards? The full 15-card sideboard runs about $46.01.
Buy sideboard on ManapoolAnnul is strong here since it cleanly counters the enchantment spells that fuel the opponent's plan: Opalescence, Parallax Wave, Decree of Silence, Attunement, and Seal of Cleansing. Replenish itself is a sorcery, so Annul can't touch it. Rely on your Counterspell, Foil and other hard counters to stop Replenish. Back to Basics is devastating against a manabase built on Adarkar Wastes, Ancient Tomb, and Skycloud Expanse, slowing the opponent's development while your 17 Islands stay unaffected. The extra Chain of Vapor gives you a second way to bounce a resolved Opalescence or Parallax Wave before it locks the board. Be careful with it, though: the opponent, as the bounced permanent's controller, may sacrifice a land to copy it and choose a new target. Daze loses value once the opponent develops Ancient Tomb mana and can simply pay the {1}, while Flash of Insight and Impulse are cut for being too slow against a deck that wants to race straight to Replenish. Keep the full Dreadnought and Stifle/Vision Charm combo package intact, since racing them before Replenish comes online remains the primary win condition.
Annul is a premium answer to their Phyrexian Dreadnought (an artifact creature spell) and Powder Keg, letting you stop their combo piece outright. Brain Freeze is a fun alternate win condition in this mirror where games can go long and both players are drawing extra cards via Gush and cantrips, milling them out while their deck gets thinner. Cut some of the slower card-selection spells since the mirror comes down to who lands and protects the Dreadnought first — speed and hard answers matter more than incremental filtering. Keep your full counterspell suite intact since Stifle/Vision Charm wars and countering their Dreadnought are central to winning the mirror.
Essence Flare can shrink and eventually kill small attackers like Goblin Lackey or Mogg Fanatic over a couple of their upkeeps, buying time against their aggressive goblin swarm. Back to Basics punishes their reliance on Karplusan Forest, Rishadan Port, and Wooded Foothills, slowing mana development for Siege-Gang Commander and Goblin Warchief. Daze is too costly to tempo away a land against a fast aggressive clock, and extra Impulse copies are cut for more impactful sideboard cards. Prioritize countering Siege-Gang Commander and Goblin Ringleader with Counterspell/Foil while your Dreadnought races them.