And Stay Down!
Perform a ground execution.

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In order to perform a ground execution, you need to complete up to Sequence 10. Once you are that far into the story, you will have the option to buy the "Ground Finisher" skill in the stealth category. It costs 9 upgrade points so it is a fairly expensive upgrade. Once you have bought it, you simply need to knock an enemy to the ground, and then execute them. To knock an enemy to the ground, keep hitting them with "Staggering Strike". This is done by holding down while in combat. Arno will shoulder bash the enemy and stun them. If you keep doing this, the enemy will eventually fall over. When they are on the floor, all you need to do is press to execute them. The trophy will unlock shortly afterwards. Here is a visual walkthrough if you are having trouble.