In one of the top posts of all time, it is stated that the maximum damage that can be done with a single action is 2,997, using Jester's Snap on 3 Nightmare cards. I would like to one-up that; 5,480 damage with a single use of the Firework card, and I could go higher; in fact, there isn't a maximum.
By fighting a Bully (who only goes 6 damage once he uses all of his 1/Fight hits), you can stall him by using Magic Shield and stacking as many dice as you want with Hall of Mirrors. Once you go past 42 dice on the screen, the remaining dice get pushed past the top of the screen and become unusable.
