If you can find 11 willing participants, playing Droid Run can net you a ton of experience points and help you rank up really quickly.
The video below will outline exactly how to do this. I haven't done this myself but I've heard that it works to perfection.
Unfortunately, it's not exactly easy to find 11 other people to play at the time you can play. It also takes the fun out of the game. But hey, it's an option if you want to power level and don't care how you get there.