Stiflenought
Phyrexian Dreadnought is a 12/12 trampler that costs one mana. The catch is that when it enters the battlefield, you must sacrifice creatures with total power twelve or greater, or sacrifice it. The deck’s entire job is making sure that trigger never resolves.
Stifle is the namesake approach. It counters any activated or triggered ability, so you pay one blue, counter the Dreadnought trigger, and you have a 12/12 on turn two. Vision Charm also works, choosing to phase out the Dreadnought in response to the trigger. The Dreadnought phases out, the trigger is now targeting a permanent that doesn’t exist, and it fizzles. The Dreadnought phases back in at your next untap step with no entering-the-battlefield trigger, because phasing in doesn’t count as entering the battlefield.
Stifle and Vision Charm aren’t just useful in the combo, Stifle can counter things like a Wooded Foothills after they crack it an go to fetch. One blue mana for “Destroy a land and make an opponent lose 1 life” is pretty good! Vision Charm can slow opponents down, on their upkeep change the lands they need to something that doesn’t help them for a turn.
The countermagic suite exists to protect the Dreadnought once it’s in play, and clear the way for you to play Dreadnought in the first place. Counterspell is the baseline. Daze, Foil and Thwart can catch out opponents who assumed it was safe to cast spell since your mana was tapped out.
The cantrip package keeps the deck finding both pieces of the combo and the protection it needs. The correct line is often to cantrip through the deck, find both a Dreadnought and a Stifle or Vision Charm, make sure the opponent has no answer in hand, and then commit with countermagic held up.
Maindeck
Sideboard
Sideboarding Thoughts
Blue Elemental Blast: Every goblin in a Goblins deck is red, every burn spell is red, every threat from a red aggro deck is vulnerable to it. Keep mana open to destroy attackers at instant speed or protect Phyrexian Dreadnought from red removal.
Chill: Against Goblins, one-mana creatures become three-mana creatures and the whole deck grinds to a halt.
Annul: Brings in against any deck where the dangerous cards are artifacts or enchantments, for example Sphere of Resistance, Oath of Druids, Mox Diamond, Sterling Grove.
Dominate: Steal key utility creatures, or steal a scary creature from a deck that doesn’t have many and managed to land one past your counters.
Tsabo’s Web: With Web in play, Polluted Delta can’t fetch, Rishadan Port can’t tap lands, Wasteland can’t destroy, and Mishra’s Factory and Treetop Village can’t become creatures. Against any deck relying heavily on nonbasic utility lands, Web cripples their ability to execute their plan. Plus it replaces itself!
Brain Freeze: In cantrip-heavy mirrors where both players are churning through their decks, a Brain Freeze with enough storm count can mill 20 or more cards at once, threatening to deck the opponent before they can win.
Tormod’s Crypt: Graveyard hate.