Perform a ground execution.

In order to get this achievement you must perform a ground execution.

1. The easiest way to get this achievement is to hold A to perform a staggering strike, then your enemy will be knocked to the ground.
2. Once your enemy is on the ground you have to hold X to perform a ground execution.

I find ground executions work better with a heavy sword rather that a one handed sword although it doesn't make all that much of a difference.
Unlocking the skills:
Staggering Strike
One-handed Mastery
Ground Execution
Does help although they are not necessary to get this achievement.