One of the fastest ways to close the game out with Grafted Exoskeleton is with Heartless Hidetsugu
It generates mana and card advantage with a theme
She costs more at 6 Mana, but adds an additional colour to her identity, and most importantly has Hexproof, which should protect her until she attacks
It takes three banished Monsters and shuffles them back into your deck while letting you draw one card
The phrase too-specific might sound like a backhanded insult, but putting down something early that synergizes with your gameplan and consistently refreshes your hand is going to provide you way more value over the course of the game than a simple mana rock ever will
Through gritted teeth I must accept, though, that its still a good bit more complicated that Hearthstone, a game which (secrets aside) doesnt allow you to interrupt another players turn