get 2 controllers, 1st controller will be the receiving character and 2nd controller is the one lending out the money (important since only progress is saved on 1st controller character apparently)
go to the shopping district and enter a store (the first one you see is the video store with scott's late fees). on 2nd controller, keep pushing circle to lend money to the first character. after you're done, press start and go to return to map to auto save the game, then quit to main menu and repeat.
to speed up the process, use the cheat code that enables you to use the same character for all players (down, r1, up, l1, triangle, circle on title screen). the code will money double for 1 character by using the same character on both controllers. after you're done, pretty much 1 of your characters will be a money bank and you'll be able to use that character to lend money to your other level 1 characters.
after you get a few thousand cash, go to that same video store and pay scott's late fees, then buy the appropriate items to max out your character in like 2 minutes. one item give out 1500 XP and the stat ones give out like +20.
also, fuck the ending credits, unskippable and it's so long that my ps3 controller turns off due to inactivity