Very easy Challenge for August
Easy way is to use one of the listed Incendiary Shotguns.
Shotgun: Burning Diplomat
Shotgun: R12K Dragon
Shotgun: Super 90 KWF
If you don't have any of the Incendiary Shotguns, you can also use any of the listed items.
...now go and kill some ZOMIE HORDES
UPDATE: Outpost Trap kills count as well... thanx time2munch79