Preeminent Captain

Declare it as an attacker and a Soldier from your hand drops into the red zone alongside it, already tapped, already swinging, and already paid for. That trigger does two kinds of cheating at once: it skips the summoning-sickness tax that would normally make a fresh creature sit out a turn, and it skips the mana cost entirely, so the bigger the Soldier waiting in hand, the bigger the tempo spike. The window is what makes it reliable. The trigger fires on declaration of attackers, before blockers exist, so the cheated-in Soldier always lands on the battlefield and joins the attack regardless of any chump block aimed at the engine itself. The defending player can still gang up on the new arrival during the Declare Blockers step, but they are now blocking two attackers instead of one, and the math has already moved against them. The real vulnerability is upstream, in the body. At a 2/2 with first strike, this needs to survive between combats to keep firing, so a single removal spell aimed at it before your next attack step is what actually breaks the chain, not anything that happens once combat starts. The demand it places on deckbuilding is narrow: every creature you want to cheat out has to share the Soldier type, and ideally wants to be attacking the turn it arrives. Meet that constraint and you get a top end smuggled into a low-curve aggro shell, a haymaker arriving turns ahead of its mana value without buckling the rest of the curve.