Note: This achievement is not glitched, it is just extremely picky. The game seems to register a KO as when you knock the player completely off the screen not just knock them off the stage and let them fall to their demise.
For this method you will need 4 controllers (You + 3 guests). Select your character as Sephi'ra and the other 3 controllers as whoever you want. Go to the map Iceburg Cliffs and move all 3 of your guests close together near the edge of one side of the screen. Now move your character near them and charge up Sephi'ra's Down X (
) move. This move has an arcing kick that has a longer range than most other moves I found. If you successfully knock all three the little ways they have to go to get KO'd you will get the achievement.
I'm not quite sure if you can get both the 2 and 3 at the same time as I messed up when I originally did it. However, if not then just set it up again with just 2 guest controllers instead of the 3 needed for this achievement.
I've attached a video of me getting the achievement for those who would rather visually see the set up instead of reading it.
Couldn't figure out the Upload Studio earlier so I added the full clip including me messing up and only getting 2.