Mindjack

I've found possibly the quickest way to level up using the least amount of time. (Less than 2 minutes, but it's all dependent on how fast you can do this!) But you'll need a friend.

Select Mission 5-2 (Tech Ninja)

Have your friend play as a Red Hacker, while you're a Blue Hacker. Have your friend Mindjack into the wraiths (speedy characters) and let them come towards you.

MELEE THEM (Tap B)

You should get 750 XP each time you do this. (Please correct me if I am wrong) Clearing the mission by this method would then give you 2250 XP.

Rinse and repeat however many times are needed. (This is especially helpful towards 50 if you haven't reached it.)