Nicol Bolas, God-Pharaoh Is an obvious heavy hitter that is hard to kill without some dedicated hate. Bringing seven damage strait to the opponent's face can easily be part of a one/two kill. In addition, all his other abilities are great grinding abilities that can help win over time if you can't scrape up 13 damage somewhere else.
Chandra, Torch of Defiance likes to grind and likes to ramp much more than what many people think of Chandra. Her first two abilities enable card draw and extra mana, both of which are important to control. But she is in the assassins crew because her ultimate ability, which can happen very fast when you can shut down everything your opponents want to do, is a one card win condition. Also her draw ability can often result in the leftover damage needed to finish the game with one or two other hits.
Torrential Gearhulk really needs no introduction for inclusion in a control deck. He is tried and true, but in this setup I tend to like him more for the extra casting/blocking. He can get in some good damage as other decks rely on him for, but we are really looking for about two solid hits to kill, and as such I have reduced the standard 4x inclusion to just 2x.
Combustible Gearhulk Wait, we reduced the blue hulk, a control legend and included a red? While this card lacks flash, which we love, it can do two other really great things: Better card advantage or nova damage upon entry. Your opponent is really forced to give you a free Ancestral Recall because otherwise it has an excellent chance of dealing one of two hits that we need to win on entry. Technically, it can kill on entry with two Commit / Memory and nothing else; it doesn't happen often, but the damage potential resting in this deck makes taking damage more of gamble than many opponents are willing to take.
The Scarab God is more of a grind enabler. He is hard to kill, he lets us filter our draws, but his ability to single-handedly amass an army earns him a spot amongst the killers.
Torment of Hailfire can be a full 21 damage to an opponents face around turn 9 (or 7 with Chandra) but in all likelihood making part of a one-two punch is more realistic. The fact this card is unwanted early game, coupled with all the draw we have, is why I have reduced it to 1 in the deck.
Ribbons is like a mini Torment but it is much more welcome early game. Late game I have found myself cutting my own guys a few times to then immediately cast Ribbons for the win. Easily part of a one-two punch.
