This achievement is currently very glitchy. You will unlock the achievement eventually by playing naturally, however, be aware that you may not unlock this straight away.
The amount of XP that is required for this achievement is 282,500.
This can be unlocked by playing Horde and/or Versus.
If you're playing Horde I'd recommend the Scout class and playing on normal (the higher the difficulty, the more you earn). If you run around and kill all the enemies--and pick up their points--you should rank up fairly quickly. Just focus on getting ribbons and you'll begin to rack-in the XP.
Turn on 'Bounties' before matches to get a massive boost in XP. From the main menu: go to Horde--manage bounties--to see what bounties you have. Don't worry, you can add a bounty before a game begins by going to the bottom right-hand corner and adding a bounty. Bounties can only be unlocked by opening crates with in-game coins.
Important: Skill is very important for versus, so if you're not very good at Gears multiplayer then the Horde option is probably better suited towards you, however, both Horde and versus work well for levelling up.
From the main menu: click on versus--social quickplay.
The thing with versus is that, if you want to rank up fast, you have to play the same gamemode a lot. King of the Hill (KOTH for short) is the fastest way to rank up, without a doubt. Aim for capturing the objective and you should net (roughly) 10,000+ points a game. The only caveat with versus is, however, the fact that getting into a game that is--and stays--KOTH, is nearly impossible. If you have a group of like-minded people that are willing to play nothing but KOTH then this option might be for you.
Escalation is also a viable option, however, be aware that it is a competitive game mode.