The Place in the Video (Durthang Outskirts Stronghold, left lower Corner on the first Map) is one Place of many where you can get the Achievement. Simple "gather" 20 or more Enemies under a Morgai Fly Nest and the shoot down this Nest.
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DrJekl write in the Comments you dont need 20 at once.
With this Info, you can now step-by-step to reach the 20!