This is by far the easiest way to boost but I haven't seen anyone do it yet, I do this when I wanted to level up fast when I wasn't 10th prestige but it is extremely easy to do and should help some people looking to level up fast.

So here is what you do:

Have 2 people ready to boost, one should have the following class, Blind Eye, Quickdraw, and any 3rd perk, with any silenced gun the other person wants you to use and a tactical insertion. The other person should use Scavenger, Assassin, and any 3rd perk. Now that you have the 2 people with those classes, one person searches for a game of Kill Confirmed, and when they get into a pregame lobby you invite the other player, this is when you decide who will be getting the kills and who will be dying (The person who dies in the end still gets 10k-20k exp per game and it gets your score high on the leaderboard because you get around 10k-20k score a game). Now you both find a spot on any map that most people don't go there and you begin to place your tactical insertion and have the other player kill you, now when you die you will land on your tag since it is kill confirmed, and will get 250 exp each time you die, or 500 when you have x2 exp. The player who is getting the kills gets their KD raised, and they help ranking up their guns and challenges. Each player gets a good amount of exp and you shouldn't get banned for this, I have done this for 4k out of my 30k kills. (NOTE: Don't stay in the same game or people will start to follow you around and figure out what you are doing)