wail on

A convenience store customer conveniently comes to the rescue. At first, the guy you saw looked like he's shopping for something to snack on. You can see now, that is not the case. He was craving what was in the cash register. But the guy in the back steps up from the booth, leaps into action, you don't want to see that coming at you. He grabbed a stepstool and wailed on the would-be robber. The guy got away, but not without suffering some serious punishment and dropping some cash on his way out the door.

wail on ---
To strike an opponent heavily and repeatedly in a fight; To beat heavily on anything