Gears of War 3

The calculator mentioned in the other solution states that for level 15, you'll need 62,125xp.

As mentioned in my other solution, I believe the fastest way to get this is in public (or private with a second/third controller) Horde mode, or versus mose matches to break up any monotony. I've yet to see anything in the game that boosts your xp total so quickly. Also, make sure you're working towards medals, as they each provide a significant boost.

Once this has popped, rejoice in the fact you're approximately 1.61% of the way to level 100! Yay!

This achievement is earned by reaching level 15 as your ranking status.

For this achievement much for XP you're able to take advantage of any gametypes for earning XP towards your level 15 ranking.

-Campaign with up to 4 friends
-Online versus, or even private versus
-Playing Horde/Beast Mode will also earn you XP toward level 5.

UPDATE: As of November 18th 2011 EPIC has granted anyone who has/hasn't started Gears of War 3 an automatic 29/30 Event Matches played. And with XP being earned at 5, 10, and 20 events you'll already have earned XP to reach level 11 from starting the game. And once you've started a match the game will register you for your 30th Event match. This will you the following achievements:

Level 5
Level 10
And once the match has been completed
Award Winning Tactics (Earn an Onyx Medal)
Socialite (30 Events played, Onyx Medal)
Level 15

However for any of these methods you'll be able to reach level 15 and earn this 15G achievement!

The following video is of my first Team Deathmatch game where the 29/30 Events played update is displayed.