Find a spot on a level where at least two robots are firing bullets at you. Try to stand in the middle of the robots so the distance from one robot is the same as the distance to the other one. Then it's just a matter of timing.
Punch two bullets at the same time with one swing, so the bullets will return to the robots and kill the robots.
It is easier to do this at one of the early levels of Normal difficulty. On Normal difficulty you can take a few more hits.
Pilgrimage of pain, Junkyard seems to be a good level to get this achievement. After the second yellow flag you pass a turned over car with explosive barrells underneath. Just after that, you have two beams hanging next to each other. On, and below the right one of those beams, you have two robots firing red shells.
Thanks h3ng3 for pointing this out!
Rourc also found a good spot to get this achievement.
You can get it on Cliffs Survival on Normal. There are many flying robots on the 1st bonus round. Some of them will simultaneous fire red bullets at you.