It's just a matter of keeping the run going long enough. The longer the run goes on, the more the kill total scales up and the more Golden Whetstones you can find. My current run is on floor 190, I've applied 11 golden whetstones to my dagger, gotten nearly 1400 kills with it, and my first attack of the fight is over 5k damage (it goes up from there because of +rage/attack from the forge). Basically, if you don't get killed you reach a point where your damage output scales up faster than enemy hp does (the Shield Spirit + Hercule Pavise combo is pretty important for this because it turns it from 2 energy per attack to multiple attacks per energy - if I cleared out the space to activate my Red Energy I could do 72 attacks per turn right now - and it gives you free block at the same time).
