Kind of like the achievement's title, your likely to just stumble on this. I just started playing this game with Density Control power, and I had leveled it up to 2 which gives you a "clap" at the end of your string to damage/knockback enemies. The Purifiers often swarm, so just keep beating on a group, making sure to keep varying who your attacking, and this should come. I believe it requires 10 or so hits strung together.