Serious Sam 4

The third mission (Four Horsemen of the Apocalypse) is the first mission in the game where the Aludran Reptiloid makes it appearance. Assuming you will be playing on Tourist difficulty it will be very easy to take down one of these enemies with just the pistol. Try to keep distance when you start shooting it. If the Aludran Reptiloid gets damage from other sources, like a teammate it might not count. You can always manually save before attempting it as you can just load the save if you need more tries. The Aludran Reptiloid will also appear more often when progressing into the game.

Aludran Reptiloid:

Load into the mission "The four horsemen" and when you get to where you have to place the first beacon and place it, the big green guy with lots of arms will spawn, only hit him with your pistol until he dies and your achievement will pop. Easiest done on tourism difficulty.