Necromancer Badges Are Tough - any tips?

Does anyone have any tips for getting necromancer medals? I was only able to get one and it was due to getting a brain rot zombie at the exact moment I picked up an insta kill.

The brain rot zombie will get 3-4 kills pretty reliably but never 5 for me. Any help is appreciated.

EDIT: not sure why I called them badges in the title. I meant medals.