Step 1: Select the 'Puppet' ability in the Signs tree.

Step 2: Travel to the coast line of Velen. (It's a great place to find groups up Humans and Drowners)

Step 3: Crazily run at groups of Humans and Drowners.

Pro Tip: Select the dark, stronger enemy type in the groups of Drowners to get the kills done faster.

Pictures of the steps below


Enjoy your achievement!