Not quickly, but putting Aether in the Great Engine is the fastest way to re-level an old character. I recommend saving up Resistance posters (at least 100) and feed them to a modified propaganda machine in the town square until you get full Aether, dump it for RP, then go back to the propaganda and refill your meter, and so on to continue leveling up.
That is the fastest leveling up method I know. Science papers can be used in a similar way if you have those by placing them on Jefferson’s desk, but it’s a little slower since you have to walk further between his desk and the Engine. Hope this helps!