Soulsurge Elemental

The single point of toughness is the whole arithmetic. Power reads off the number of creatures you control, so a four-mana investment buys nothing into an empty board and a game-ender into a full one, but the */1 frame means the same token-swarm deck that powers it up shares its weakness to any one-damage answer and any sweeper. First strike is the detail that makes the variable power credible in combat: the blocker eats the first-strike damage before it can hit back, so this can connect for five or six and survive a block it would otherwise lose. Crucially, it is a single tall threat rather than a team-wide effect. It pumps only itself, so a lone chump blocker still stops it cold, and it asks your other creatures to clear a path rather than benefiting from it; that puts it in the family of self-referential bodies whose stats count your own board instead of a one-shot anthem that deflates at once. The trade-off is honest in both directions. Develop the board first and it arrives as a finisher that doesn't need a pump spell to threaten lethal; cast it into nothing and it is a worse-than-vanilla beater waiting to die to a stray point of damage, with no recursion to soften the loss once it goes.