The description is misleading. You don't need to air juggle enemies and pummel them while in midair, it suffices to mash with Galen in a group of enemies and most of the time you score some hits on airborne enemies without recognizing it. Another good is way is to level the skill "Quake" and use it in a big group of enemies. Even with Quake on level 1 it is a piece of cake, it just takes a bit longer and the hits don't have to be consecutive. You may earn the achievement in Galen's second or third mission.