Starfield

There is a perfect way to do this and you can do it right away, in a few hours.

Couple of small tips.
Obviously, the stronger you are the better.
I'd say bring a Melee weapon so you don't run out of ammo. Investing in the Dueling skill is helpful but not necessary.

Anyway, let's get to it.
We will be visiting the Jemison planet in Alpha Centauri System. That's where New Atlantis is but we're not going there.
Instead, just find any place on the planet where the destination will read Mountains.
We don't want Deciduous Forest, we need Mountains.
The reason we are choosing Jemison is because it's a set planet (always the same) and because it's a lvl 1 planet, which is the lowest in terms of difficulty.

We are going to find 4 species of creatures here.
Out of these we want to farm just one and that is the Herding Cutterhead Herbivore
Like the name suggests, they won't attack unless provoked but thankfully they are complete pushovers.
Even better they are always in packs of 5-10 so this method is going to net you the achievement in a few hours.
Even the hardest ones will fall in a matter of a few stabs.
If you are having trouble you can also lower the difficulty to Very Easy when doing this.

Out of the remaining three, there's a frog type creature called the Hunting Tuskfrog that you won't see much of.
Feel free to kill it.
And then there's Pack Coralbugs and Apex Parrothawks.
The former can be very dangerous when in packs but relatively safe to kill if alone.
However if you spot more than one, better leave it alone.
The Apex Parrothawks are, by far, the worst.
Not only are these guys really dangerous, they can and will steal many Herding Cutterhead Herbivore (HCH) kills.
They are also really agile. Honestly, these guys make Cliff Racers look like a Sunday stroll.

Regarding HCHs, their attack is laughable and they might even try to run away but you can catch up to them with your booster pack.
If you are starting to see the bodies of the creatures you have killed, just jump to another Mountains spot in Jemison and do it all over again.

You can view how many you have killed in the Pause menu, then Y and under Combat you'll see how many creatures have been killed.
NG+ does not reset kills.

One last tip that is extremely important.
Because you will be on one planet for a long time, this means no fast traveling and no screen loading.
So no auto-saving.
Absolutely make sure to save your game every once in a while because you don't want to have to do this more times that you need to.

Happy hunting!