The easiest way to achieve the Gold medal is using boomsticks, secondary attack with combo card. Two of them should be enough.
The number of zombies on the second floor is reduced once the challenge starts. However, the first floor remains crowded, and you can gain PP killing zombies on the first floor as long as you stay on the second. Using three to four Hail Marys on the crowd on the first floor makes this challenge extremely easy, as you can achieve Gold medal by this way within literally half a minute.
The name of the challenge may be a reference to the survival-horror zombie game "Killing Floor".