First, make sure that your gun is loaded with fire bullets (See Fire Bullets for more information). When a Humanz attacks you, you’ll see glowing orange lines in its skin. You need to shoot it directly in one of these lines. Usually there’s one that’s significantly larger and wider than any of the others. You might get lucky and just hit one, but there are a few things you can do to increase your odds.
You can try knocking them down with a melee object, so they’re not a moving target, before running up with the gun for a closer shot. You can also wait until you’ve burned several Roots of Evil. Burning the roots will give you Spectral Vision. Once you have this, you can close your eyes and see the fissures glowing white, which makes them much easier to hit.