I tested his Hammer Smash on both flash and Steam.
Steam
flash
on Steam, it deals 60-120 'true damage' with 8.5 seconds of reload time (once after every 7 normal attacks).
on flash, I'm not sure about its exact damage output but have seen him once dealing 238 damage. and the cooldown is slightly longer; 10 seconds (once after every 8 normal attacks). Also, the ground-rupturing area is bigger than on Steam.