By far, the easiest way to do this is in Firefight on Courtyard. This map is the same area as the first section of ONI: Sword Base, and has two forklifts. Choose Gruntpocalypse as the gamemode on easy difficulty. Change your weapon loadout so you spawn with a Gravity Hammer with infinite ammo. Also make yourself invulnerable in the Base Traits section of the options. Now, simply run around the map and smack the forklift around with the gravity hammer. When you hit it, it will fly forward with a lot of speed, and can take out entire groups of grunts very easily. I was able to get a splatter spree within a minute of starting with this method.