Boot up Lifeline.
Jump in a car and drive to areas with zombies.
Sword/machetes/knives are what you want to use. Weapons with nails or axes don't count. (Probably killed a good fifty or so with a axe before checking to see I had no progress gained.)
Every two kills will get you one percent.
This isn't hard at all as long as you're moving around efficiently, should be able to get this done in under two hours.
Make sure to bring multiple weapons in case the one you're using breaks.