If you use a gold coin in the Nexus before talking to The Maiden it glitches your luck stat to keep ticking up to a cap of about 84,000. This luck stat does work by the way and you are guaranteed rare drops from any enemy you kill.
While you’re in this glitched state if you do a password invasion with somebody they can use Soul Sucker on you to get all the souls that you would have used to level up. At the cap of 84,000 it actually rolls back around and only gives you zero but at around 30,000 to 40,000 you end up getting close to a billion souls. This also knocks the player in the glitch state out of it and their stats go back to normal. You can do this once per gold coin.
Using this you can level up to the cap of 712 in about two Soul Sucker runs. I’ll update this post later with some screenshots and maybe some more accurate numbers.