When an enemy is about to hit you, hit to counter the attack. Do this three times in a row. If you do anything else between each counter, you have to start over. The button prompt goes by far too fast to do on reaction, so base your timing on the animation of the attacking enemy. Zombies are easiest because the animation when they're winding up for an attack is slow and easy to see.
An easy place to get this is in the Urban Chaos stage in Mercenaries mode. At the very start, there will be two weapon wielding zombies. Counter them with and then find a third. If you mess up, you can restart the game instantly to try again. Also fun note: Your progress is carried over even if you restart. You can counter the first two, restart, and get your third off one the of zombies at the beginning.