Mizzix of the Izmagnus

The threshold clause is the whole engine: you earn a counter only by casting an instant or sorcery whose mana value exceeds your current count, which makes accumulation a sequencing puzzle rather than a reward for casting big. The optimal climb is a staircase: a one-mana spell at zero counters, a two-mana spell at one, and so on, each cast clearing the bar by exactly enough to advance. X spells short-circuit the ladder, since their mana value on the stack scales with whatever you pour into X, but a deck of expensive spells with no cheap stepping stones stalls just as badly as a deck of pure cantrips. Once the count is high, the second ability turns the deck inside out: the discount has no cap and shaves a generic from every instant and sorcery you cast, so a board ten counters deep stops resembling a normal spellslinger list and starts behaving like a combo deck waiting for the turn its X spell goes critical. The catch that keeps the design honest is that the experience counters live on the player, but the cost reduction is a static ability on Mizzix's own body. Kill the 2/2 and the counters persist, yet the discount evaporates until she returns to the battlefield. That split (permanent progress, fragile payoff) is what defines the deck's whole posture: protect the commander, because the counters are stored value that only the creature can spend.