Back before there were different spawn points for the teams, they spawned at 1 block in the map. It was easy to get multiple kills right at the start
Also, back when destroyer was working on ghost detection I had created a ghost of myself for him. I then went on the other team and placed a zombie next to my ghost. Got over 100 kills with that 1 zombie.
Lastly, I had combined portals and a zombie. The enemy would go through the portal, be placed in the zombie and then would die. Got so many kills with that combination
EDIT:
Those who placed the zombies on the spawn got a lot of kills as well but ya... the players would spawn, die from zombie, respawn, die from zombie, et cetera.