This achievement can be done in either player or ranked. In order for your kills to count towards the achievement, the match must finish. That means you need to set a time limit and/or a frag limit. If the time limit and frag limit are both set to unlimited, the match will never end and your kills will NOT count for the achievement. Because of this you should avoid choosing the "quick match" option as it usually makes a game with unlimited time and frag limits. So long as you see the end of match tally screen, you know your kills counted.
If you are boosting this:
- The fastest map to do it on is E2M9: Fortress of Mystery.
- Set a high time limit(but not unlimited) and an unlimited frag limit to get the most kills per match
- Kills don't count if the connection drops, so many players recommend setting a kill limit or low time limit to minimize progress lost in the event of a dropped connection