It's pretty easy to get this without trying in single or multiplayer. You need a rampage with 5 kills. Aim for a kill a second. In single player, the first Aspari level towards the end has quite a few opportunities to get this, even on hard mode. In coop mode, there are a ton of opportunities to get this if you play on normal by yourself. If you're having trouble, level your character up, fully upgrade the sniper rifle, and let guys pile up. The sniper will rip clean through a group of people, and you have 8 rounds to play with.