The First Descendant

The fastest way to level up in the game is to play the Albion defense mission in Kingston or Echo Swamp. These are horde mode missions and you'll get experience (as well as weapon mastery) even if you don't damage an enemy, so you'll often have high-level Bunny characters running around killing everything while you rack up the XP. Most people quit after Wave 7 or at most Wave 10 because the difficulty really ramps up after that, but just queue again and you'll find another party right away. Doing this you can go from Level 1->40 in like two hours with any new Descendant.