In order to unlock this achievement (I unlocked it on 31th March), you need to get
50 killing blows (not normal kills). A kill blow is obtained when your gametarg appears in the screen when an enemy die (you will also hear a sound in game), you have to be more careful when your opponent has very low health because even if you dealt 99% of damage, the game will count the killing blow to
the player who give the last hit. I lost a lot of killing blows due to this. You can press
during the match you can check the stats and see how many kills and killing blows you did (there are also information about heals and damage dealt).
You don't have to do all the kills in a single match, you can do in several matches, but you
DO NOT have to close the game in order to count them.
In my case, to unlock it I've played 2 matches doing 15 kills in total, then I went to the Dojo (I do know if it is useful, I read it in the comments of other guides and I tried), then I played other 7 match and during the 7th I unlocked the achievement during the game. Another things that I did was to
NOT PUSH 
to return to the main menu, but I waited that the games automatically returned in it. After every single match, when I was in the main menu I checked the achievement progression and I saw that after the 6th match it went from 84% to 96% and then I unlocked it during the 7th match.
If you are interest in the characters used, for all matches I used Cass but for the last match it was already taken so I used Kiev. I don't think that those characters are the only ones who can unlock the achievement, but I wrote all steps that I made in order to unlock this. In my case, the achievement was stuck on 84% for over 3 days, having in the profile counter over 500 kills.
I hope devs will patch this soon, in the meanwhile you can try to follow my steps. Good luck!