If you are like me, then you are playing this game after it is rather old. I didn't have any friends to allow me to gain experience off their character, and when I made boosting sessions, no one responded. So I found an alternate way to mooch all of my characters up to level 80, and this will allow you to reach level 120 with one of your characters as well.
What you need to do is beat the game twice with one of your characters to unlock extreme difficulty. Now when you load up the game, choose that character start a game on extreme. Once you have your game saved with that character on extreme difficulty, you can view the available multiplayer lobbies, and it will only show you the ones that are on extreme difficulty. At the time of me unlocking the achievement, there were almost always a few lobbies where people were playing through the game on extreme. Also most of the time, these people had modded weapons that allowed them to kill everything easily.
So here is what you do. You join an available lobby with your character that has extreme difficulty unlocked, then leave. The people that were in that game will show up on your recent players list. If they are not appearing offline or away etc, then you can join their session in progress. When you do so, it will take you to the character selection screen. Now when you are here, you can choose one of your low leveled characters and bring them into the lobby. If the people don't mind (most didn't for me), then you can just hit the enemies once and allow the others to finish them off, and you will level up very quickly. You will have to start a new file with a character before you can use them in a multiplayer game, so don't try to join a session in progress then create a new character.
Hope you find this helpful! Let me know if there is anything that could be improved upon.