Vampire Rain
20 Achievements
1,000
40-50h
Nightwalker Killer
Scored 500 or more Nightwalker kills in single-player mode.
50
How to unlock the Nightwalker Killer achievement in Vampire Rain - Definitive Guide
OR
Replay Mission 23 ( Charles Boss Battle) ~50+ Vamps in 5-6 Minutes. While this Method is a bit faster though the Downside is you have to play the other 22 Story Missions to Unlock it.
Edit: Just Added Plasma SKX's from NTA Amazing Video Guides so all Credit for the Footage goes to him :)
You need to kill 500 Nightwalkers. If a nightwalker is engaged, it will take many, many rounds to take them down, and if there are any nightwalkers in the area, they will be attracted to your attack on their friend, and will kill you very quickly. The only way to not attract other Nightwalkers, and to kill your prey quickly, is to use the UV knife.
One easy way to get this is on the boss fight with Charles. You get tons and tons of ammo for your light 50 caliber sniper rifle and the enemies just keep coming. If you die, just retry. The achievement pops as soon as you get 500 kills.
You may also unlock it just through attempting "Real John Lloyd" and "Collector", as you will be killing many Nightwalkers with your UV knife.