This is really simple and only takes a minute.
Load up mission 18 'Thunderhead' as you start with the tank. Drive to the mission objective furthest in the east.
Once there, you'll see a ton of spawners, and a ton of zombies. The spawners just make more.
Just stop, and let the zombies hit you. Back up against a wall if you can.
Keep letting the zombies hit you until your on about ten percent health. You'll see that there are now that many zombies surrounding you, they can't get any closer. Before you die, don't forget to fire.
I got 96 on my very first shot, so should be easily doable for everyone.