This is a guide on how to solo boost this method if you do not choose to waste more money on an average game just to get your rank up and achievements unlocked.
This comes from X360A known as the "Aftermath Gondola" method, all credit where it's due to dkykong and co, I'm just writing it here so it makes sense for those who can't understand his explanation.
This method is quite similar to the Act 2 Chapter 7 boosting method for medals in Gears 3, where you reload a checkpoint before a cutscene.
You'll need two controllers (or more if you like, but two is optimal as explained below), only one XBL account is needed, guests for the rest. You'll also have to be able to play Judgement on insane, so finish the campaign once, obviously.
Play through Aftermath's final level on insane until you finish the kitchen and the gas/grenade cutscene starts. go back to the main menu at this point and sign in the second or more controllers. Now, go back to the aftermath campaign and "continue". This should start the cutscene for the kitchen and gas/grenade again. Feel free to skip this cutscene now and you'll be riding the zip lines down while the ship tumbles beside you.
Your main character will then have to shoot all the gondolas for any extra controllers you signed in, once you land on the bottom, you'll hear Baird ask a question about being able to steer the boat, the screen will fade and you should (but not always) see the save prompt come up, hit START as the screen starts to fade to black and then reload checkpoint. It'll take you back to the kitchen cutscene again and score you 750XP for every controller you had signed in. This is where number becomes important, if you're using more then 1 controller you have to shoot out the gondolas on THEIR lines as well as yours or you die, reload cutscene and get no xp. You'll only get the XP once you land safely on the roof below
Each run through takes approximately 2 minutes, so you'll get 7500XP every 10 minutes without having to spend ANY more money or in a large boosting group. Even at the level 40s you're only going to be spending about 10,000XP to level up roughly (I think it's 12-15k by level 48-49) but this method will score you a rank up every 10 minutes easily. Rinse and repeat as much as you want.