If you're going for the Real John Lloyd Achievement you really don't have to worry about this Achievement at all , but in Case you don't like the Game there is a fast & easy way to earn this Achievement Early in the Game , just keep playing Trial 5 ( Shotgun Trial) you can easily Rack up 15+ kills in 2 1/2 minutes.
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 :)