A good way to do this is to target the legs of a Screamer or Crusher until their balance bar is drained (the blue bar underneath their health). Trying to do this to a Slobber is a fool’s errand, as you’ll get vomited on - it’s better to keep your distance. Do this to Screamers and Crushers by hacking at their knees ten times, wearing down that blue balance gauge, and this will eventually unlock. Curveballs like the Shuriken are also good for whittling down an Apex's balance, so experiment with those, too.