Gain Experience Faster By Engaging In Story Mode
Story mode is probably the most reliable way of gaining large amounts of experience. I don’t know how accurate this is, but it feels like experience gains are linked to how long you play and not necessarily how well you play. Of course, challenge completions are very good for leveling up, so keep an eye on challenges that are close to being completed. Fast games of PvP might seem like they're good, but given all of the down time between matches and the very low amount of experience you gain for a fast game, they're not actually that effective.