There are ways of doing this and both are fairly simple and allow you to boost headshots nukes attachments including FMJ's.

Normal FFA boost Locked Lobby.
- The first step is finding three people you can TRUST that won't mess and around and screw things up
-Second is the proceedure:
-start a free-for-all match and while searching click up twice to the invite option go over once to invite party to game, your party members should be in their message boxes ready 2 accept any incoming game invites so they can get into the game immidiatly.
-once all 4-8 of you are in the game( if done correctly) the lobby should now be "locked" as the online term is, meaning no one can join your game unless you send them an invite; this also includes joining session.

The second way is for attachments and headshots only as their are no killstreaks:
-start a barebones pro lobby(the one with free-for-all and TDM) and if the host of the game is NOT the one who started the search then the lobby will be locked and do as above.

**NOTICE: Locking a lobby may take several attempts and may annoy some people but I garentee it still works because I have Hosted several Boosting Lobbies in the past few days -- The host of the game MUST NOT be the one who started the search or the lobby will NOT lock -- This may require that the host leaves the lobby and is reinvited quickly so that the lobby will be locked.\

I hope this has helped any boosters who want to get their emblems, challenges and weapon attachments/levels without the constant harassment of enemy players.

All credits go to my online friends who taught me how to lock a lobby I'd add their GT's but i no longer have them added due to a new XBL account.