No sideboard changes are recommended for this matchup. Phyrexian Dreadnought is always a 12/12 trampling artifact creature. When it enters, its controller must sacrifice it unless they sacrifice any number of creatures with total power 12 or greater. Their deck has essentially no other creatures, so the Dreadnought only sticks if they Stifle the trigger. Pyroclasm's 2 damage can never kill a 12/12, and there are no other opposing creatures for it to hit, so it stays in the sideboard. Swords to Plowshares remains the best single answer to a Dreadnought that sticks, since exile dodges any further shenanigans (they gain 12 life, but that rarely matters). Armageddon is excellent here, destroying their Islands before they can chain cantrips and counters, though it can still be countered, so sequence it with that in mind. Call of the Herd stays in as a source of board presence and card advantage via flashback.