Throughout the course of the game you have a good chance of unlocking this, however if you need to grind this out, Episode 7 is the best place. At the beginning of the mission you'll fight a few waves of regular and elite enforcers. Don't waste your time trying to do this on the elites as you will probably get killed just trying to chip away at their health.
Also, you are allowed to hit them with other attacks, just the killing blow has to be with the Spammer. You'll take out about 5-7 enforcers in the start of Episode 7, just make sure you activate the elevator before you exit out of the mission so your kills will count.