Survive through the 11th wave (on the Colosseum map at least) without losing any of the heroes and capture both Strategic Points as you finish it. When the 12th wave starts, your multiplier should be 324x.
Use mobility abilities such as teleport or jump to kite the waves to death safely. Focus on not dying, ignore time and the Strategic Points until you can finish wave 11.
Gaining this achievement unlocks the Laughing Stave.