Just like the machete counters, except you have to do it on an enemy holding a gun. For these ones you don’t have to knock down their health before you can perform it. Just wait near an enemy with a gun and when they start to aim at you and the red exclamation appears press near them to perform the counter. This is cumulative and can be done in story mode and survival.
A good place to grind these out is about halfway through level 3. You'll come to a point where there's a guy trying to squish you with a boat. You have to kill him before moving on so you can just keep performing counters on the enemies that jump in until you unlock the achievement.