Become Immense and Temur Battle Rage are a couple of cards from Khans of Tarkir block that lead to some quick kills. Together, they will one-shot any opponent, even if they're still at a relatively high life total.
The real combo is Death's Shadow with either of them, as a Become Immense while you're at one life will be good for eighteen damage, and a Temur Battle Rage when you're at three life will be twenty points of trampling double strike damage.
Cards like Street Wraith and Gitaxian Probe do double duty, helping out your Death's Shadows while also filling your graveyard for delve. Patrick Chapin's Mishra's Baubles also make an appearance. Monastery Swiftspear, Steppe Lynx, and Wild Nacatl are the biggest creatures for one mana, which gives you the best chance of one-shotting your opponent.
The sideboard looks kind of rough, but it's difficult to sideboard in a deck like this. You're trying to assemble a certain amount of resources to combo kill your opponent, so any cards you bring in dilute your deck overall. I think Thoughtseize is a good start and Phyrexian Unlife might be great against other fast decks, but past that, what do you really need? Leyline of Sanctity might be nice against opposing Thoughtseizes and Liliana of the Veils, so you could try that.