Not as bad as you'd think. If you don't have this after 5-skulling everything and doing Nightmare, you're close. I used the penultimate level, "Stairs of Doom", to rack up kills because it's easy. Just fill the two choke points on either side with three clockwork maces (upgraded swinging mace) and a 3 x 3 grid of tar traps under them, and waste whatever money you end up with afterwards. This should net you about 700 kills each time. If you can keep up with the kobold runners in the early waves, this strategy also works great for 5 skulls and Nightmare mode.