There are three ways of knocking the opponent:
1) When your robot is more powerful, you'll make a heavy blow and can knock down your opponent. You must time this correctly or it will miss.
2) Your machine is pretty good but not quite reliable, once it lands enough blows on your opponent's body, you will get a chance to use special attacks. Use this opportunity because if your opponent blocks the attack, then you will be in trouble
3) In addition to these two methods, there is another way of getting a knockout. I do not know how this battle system works so I have tried many times with different strategies but always fail even though my machine has more strength than the other one.