This achievement will very likely be obtained by naturally playing through the game!
If you however want to unlock this achievement very early on, a good point to farm this is in the LOST TUNNELS
level. You will encounter a lot of Nosalises during this level, especially at the end where you need to hold your ground in front of a gate and wait for reinforcements.
The only downside is that your comrade, Bourbon, can steal your kills with his AK/Semi-Auto Shotgun. Play trough the level normally and focus on actually dealing the final blow to each monster. At the end of the level you will be attacked in waves of about 20/30 Nosalises total. Perfect for farming kills. Hit checkpoint reload as soon as the enemies stop spawning in, this is when Bourbon tells you to walk into the light "because these guys have no sense of humor".
The progress does not reset if you reload checkpoint and you can farm this achievement quickly. This is also a great place for these other achievements:
There are pistols and shotguns scattered around the level. However if you want to make 100% sure you have the right weapon load up the RIGA
level right before the Lost Tunnels and buy the weapons from the vendor before heading out to the surface(The vendor is directly under the bar from where you start the level).Like I mentioned at the beginning though, there will be more than enough enemies so don't worry about this achievement. It is harder to complete the game without obtaining this than with obtaining this.