This achievement will most likely come naturally as you play through the game but if you wish to grind it out quickly then going to the rooftop (level 4) gives you an easy method to quickly rack up some kills. By setting the dance floor on fire you are guaranteed 20 kills with potentially up to around 30. Afterwards I recommend walking in to the fire yourself as that would cause you to die which helps towards the
Unbowed achievement. Plus you will get the kill x amount of people in a row achievements too.
Here is a video guide of me doing this if you would like to watch.