You need 50 EXP in one playlist for this and the quickest way to do it is to boost them.
You need one person as a quitter (must have 4 controllers and an useless account).
You can boost 4 accounts at the same time (they don't have to be on one console).
First you need to change your language for the quitter and the booster party leader, it works best with chinese (the very bottom one).
On the bottom you see a vid how to change a language, so you won't have any trouble changing it back to your language. (it's the old Dashboard in the vid, but works the same)
When you have your language ready,
Booster party leader: go to Halo 3 Matchmaking lobby, then invite your friends, who also wants to boost this, or turn on your guests. After you have 4 players in your lobby, go to Social Skirmish or Team Mythic playlist (Must have all DLC) then press "X" and select the bottom one from the list. The quitter has to do the same (turn on guests, go to the playlist, press "X" and make sure you have to bottom one always enabled).
If you have done everything correctly, it should always find your boosters and every time just quit with the quitter, so your boosters gain 1 EXP per match. You can boost as much as you want, (for example boost rank or playlist rank) without getting banned. You can also do some other Achievements while you're doing this, but I suggest to get EXPs first and then boost the other Achievements, because it can get you banned from earning EXP.
There is many different methods for this, but the fastest is:
1. chinese language
2. 2 consoles, 4 controllers on the quitter's console and 4 controllers on the booster's console.
3. 3 guests on both consoles.
4. Social Skirmish
5. quit fast and start the game fast.
You should get this done in 1 hour. Good luck and hope this helps. ;)