Basically, you earn exp pretty fast in the campaign to go towards your online rank. I haven't seen this thread anywhere else on TTG, so I will post it here. I will post the steps so you can then do it for your self.

Don't complain if it doesn't work, it's probably because you did it wrong, so repeat the process if it doesn't work.

1. Go into ranked match
2. Select DM
3. Press Search
4. When it says 1 Player Need 7... or something like that, back out into the main menu.
5. Select Single Player
6. Select Marine Missions
7. Select the 2nd to last mission
8. Play it & beat it. (On Normal Difficulty)
9. Back out into the main menu
10. Select Multiplayer
11. Create a player match lobby to see your rank
or 12: If step 11 didn't work, go into a ranked online match and finish it. Your rank should then update.

You can do this as many times as you want.... It's an easier way to rank up.

NOTE: This glitch only updates your multiplayer rank. In saying this, it does NOT update the leaderboards.