This must be a RANKED match and you must win (DM, ABD) or be on the winning side (CTF, TDM)
You need at least 4 people to start a ranked match.
Can be done in any of the gametypes
-Deathmatch
-Team Deathmatch
-Abduction
-Capture the Flag
You will obviously get this when going for the 50 wins:
Capture the Flag is the fastest gametype (only requires 3 captures to win) and will also help towards your 50 flags: