Slimy Piper

A threshold switch bolted onto a combat trigger, and the count it keys off tells you exactly what deck it belongs in. Attack with a thin board and the bump is modest; attack once you already control four or more creatures (this one included, so it needs three friends to flip) and the trigger upgrades to a bigger swing plus indestructible for the turn. The conditional indestructibility is the interesting part, and its timing shapes how you use it: because the grant lasts until end of turn rather than only through combat, the creature walks through blocks, survives destruction-based removal aimed at it mid-combat, and stays safe through your post-combat main phase and end step. What it cannot do is stop a sorcery-speed sweeper cast on your opponent's turn; the protection is a window that closes when your turn ends, not a standing shield. The condition also lines up with the state a go-wide green deck already wants to be in, so it rewards the plan you were pursuing instead of demanding you contort around it. With a bare board it is a fragile 2/1 that swings a hair better than its body; as the fourth creature in a developed one it becomes an attacker that shrugs off destruction-based removal while your turn is still running. The floor and the ceiling are both honest, and the distance between them is exactly the gap a full board is meant to close.