Sylvan Safekeeper protects your own combo pieces from targeted tempo plays. Sacrificing a land to give Hermit Druid or a resolved Phyrexian Dreadnought shroud lets you blank Chain of Vapor, and it also stops Vision Charm from phasing out your Dreadnought. It does nothing against Powder Keg, which doesn't target, so don't count on shroud to save your creatures from it. The extra Stifle is flexible. It can counter the activation of Powder Keg to save your board, and it can counter your own Dreadnought's enters trigger. The Dreadnought then stays on the battlefield as a 12/12 trampler instead of being sacrificed. Do not use Stifle on their Dreadnought's trigger, because that would let them keep their 12/12. Duress comes out because it trades poorly into a deck full of cheap cantrips that reload immediately, and tempo matters more than one-for-one discard here. Race aggressively and keep Stifle available for Powder Keg or your own Dreadnought trigger.