Rickety Bridge

Fading 6 (This permanent enters the battlefield with 6 fade counters on it. At the beginning of your upkeep, remove a fade counter from it. If you can't, sacrifice it.)
Whenever a creature attacks, flip a coin. If you win, exile that creature. If you lose remove a fade counter from Rickety Bridge.
Creatures with power greater than the number of fade counters on Rickety Bridge can't attack.