You guys may or may not believe me about this because I am a choob or w/e. But try it out anyway if you want.
Go to: https://www.mpgh.net/forum/191-call-d...ml#post1595422
and follow all of Koeninatorrrs steps but from Step 6 its a little bit different. Get a class and customize the weapons to something that you don't have, but now you do (because of the Trainer). Then go play in-game and try and get one achievement for that weapon. When you do, go all the way back to the main menu for multiplayer and agree to become prestige. Then your stats will be saved.
I'm sorry that I didn't post any screen-shots or anything. This may also work for other trainers, haven't really played around with them yet. But Koeninatorrrs works for me.