An Over Kill is when you kill an enemy with a lot more damage than they have health. So weakening an enemy down and then hitting with a big tech move would most likely give an Over Kill.
This one should easily come by just playing. Especially as you get stronger. Personally it came when I used the combo "Blowbeat". The combo is having equipped Provoke on Nidr and Shock on Ender. I was against about 5 enemies and I used the combo as my first hit, with Momentum, and got an Over Kill on them all. I was early level 20s