To unlock this achievement, you must win 20 matches, the gametypes you can unlock this through are;
Infinity slayer- Team gets 60 kills overall to win
Big team infinity slayer- Team gets 100 kills overall to win
Dominion- Capture 3 control points and eliminate enemies
Regicide- Come in first, first place also has a bounty when killed
Flood- Infection flood kills spartans, they turn to flood
CTF- Capture the flag, capture 5 flags to win
Oddball- Get the most score when holding the ball to win, team game
KOTH- Get the most score as a team to win
Team slayer pro- Standard 4v4 no radar 60 kills to win
SWAT- One shot headshot, 60 kills to win
You must join from the beginning or it won't count, but will add to your total, thanks General Joury for confirming thisYou must be first place in a free for all game as wellGood luck- if you spot any holes in the guide either send me a PM or drop a comment below :)
You can check wins at;
https://app.halowaypoint.com/en-us/Halo4/Under war games: matchmaking