This achievement is obtained for killing 300 Human enemies cumulatively during your playthrough. As you progress through the game and story these kills will stack up and the achievement will pop during a fight most likely. Enemies such as bandits, Natives, guards, etc count towards these kills.
If you'd like to get this done quick without waiting for natural progression you can find bandits inside cities like New Sérène and San Matheus at night. There's normally 2-3 spots where a group of 3-4 bandits spawn during night time. You can go and kill the bandits at this spot, return your house, sleep for 24 hours to trigger the next night, then kill the bandits again. Rinse and repeat this process until you've killed 300.