Okay So I got this at 75 kills on an Offline Team Objective Match.
Now I know this is very glitched, as I got 54 Kills in a "Beginner Mode" Match, and 57 Kills in a "64 player Mixed Modes" Match, neither of which popped the achievement.
This is what I did from clicking play -
From opening Chivalry 2, I played a Bot Match in "Offline Practice Mode" on Tournament Grounds - With max time limit and max tickets, I was Agatha, and I got 87 Kills, and played through to the end of the match.
The Achievement did not pop.
Then I went back to "Offline Practice Mode" and stated a Seige at Coxwell with default 32 Bots in Game.
It did not pop at 50 kills, and I had to play through all the way to the end and almost killed all soldiers before it popped at 75 kills, I ended at 80. Achievement Popped.
There is a point where you msut break through a gate to reach the interior wall and your troops will only attack it from inside, That parts hard and takes up most of the timer, I had 13 seconds left when I breached the gate.
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If I had to surmise a plan of action here Id say play offline bot matches repeatedly until it pops, and just go on rampages well past 50 kills.
Goodluck!
Edit* 2023 June
Still Bugged, Highly Suggest Trying The Above If You're Looking For A Chance For It. Good Luck!