To do this quickly, you need to set the difficulty to EASY and then run to the flag in the middle and raise it.
The first wave of weeds are very easy to kill. It is best to use a zombie with a decent spread or fire shot (the scientist were great) and sit just outside the circle by the pot allowing the weeds to capture the circle as you kill them.
What you want to do is leave at least one weed alive to take the flag at the end of the round and you can quickly raise the flag again. Each round takes around 1 min to get 39 kills, netting you 2000 an hour.
DO NOT KILL THE LAST ONE. It takes too long running back to the base and waiting, almost doubling the time it takes to get the same number of kills.
NOTE - You should do this at the end considering Epic Quests generally require you to get kills.