Wall of Resistance

A blocker that grows by getting hit: the end-step trigger only fires if the wall took damage that turn, so every attacker that swings into it and lives makes the next assault harder. The +0/+1 counter is the design conceit, a defensive inversion of the aggressive growth on creatures that bulk up by dealing damage. Flying plus Defender means it answers the air and the ground at once, and the toughness-only growth keeps it firmly in its lane: this is a wall that never threatens to become a clock, only a deeper and deeper hole for an attacker to throw bodies into. The conditionality is also its weakness. It has to survive the damage to bank the counter, so a single removal spell or a large enough attacker erases the investment before it pays off, and against a board that simply declines to attack it stalls at 0/3 forever. The whole engine runs on damage being dealt: the wall rewards any source that hits it and grows off that incoming damage, an attrition piece that turns incoming pressure into the resource it feeds on.