If you are unsure on how to Nemesis Boost, be sure to check out this tutorial : [ Register or Signin to view external links. ]
Credit goes to Ramboslice/cl0ckw0rx for putting together that in-depth tutorial.

Here are the steps on how to perform this 'Glitch'.

(1)Go to the Xbox 360 dashboard with the Battlefield 3 disc loaded.

(2)Sign out of your current Xbox Live account.
-Do this by hitting the home button on your Xbox 360 controller and hitting (X) to sign out.

(3)Load Battlefield 3.
-Do this as you would normally load your game to play online.

(4)Wait until the start screen opens, press start to continue.

(5)You will be prompted that you have no active account and that you need to sign in your game, decline this by pressing the B button.
-After hitting the B button, you will be able to select the Multiplayer Option, hit the dashboard and sign in. You want to select Multiplayer.
-After selecting Multiplayer as you normally would, you will be prompted that you require an update. You also want to decline this update by pressing (B).
-After declining this update you will be able to load the server browser, and the quick match function, this is were you want to sign back into your Xbox Live account.

(6)By doing this, you bypass the patches and are able to put your game-save back into the original content of the game before the patches were created.
-This will allow you to nemesis boost without the cap of 200 nemesis score which was introduced on the second patch.
-Allowing you and a successful squad of boosters the ability to earn millions of experience pair game.

Important Information:
-In order for you to boost with your friends, they will also have to do this in order for all of you to be playing on the same version of the game.
-Doing this does not DELETE your patches that you have downloaded, you simply have to dashboard and re-enter the game in order for you to play on the most recent version of the game.
-Your information transfers from the older version to the newer version meaning that your experience is saved.
-In order to find games, (due to the lack of servers on the server browser on the older version) you will need to use the "Quick Match" function.
-In order to set-up a boosting lobby, search under the quick match option "Seine Crossing, Squad Rush". or a variant of maps.
-Although I called this a Glitch, I'm 95% certain that you cannot be banned for this, as technically your game comes in this un-patched state straight from the box, given that you declined all the updates.

Be sure to leave a response if this helped you