1) get a streak, if youre decent enough a 50 game streak will give you 10x multiplier for an hour.
2) buy an xp multiplier for an hour in te archives.
3) turn off the game time, go to hall of justice in versus and pick flash.
4) to the right where the ceiling car interctable thing is. Hold jump and interactable button the let go. Do this over and over again.
5) get 50 million exp in an hour.