Dear Matt,
I am sorry but the main way to get a lot of gold is going through the single-player levels. Like you did type in your first post, you could do Multiplayer, but it is pretty hard to not have the others steal your gold...
You could go to the level that you obtained the most gold in, and replay that for a while. Though I recommend just continuing through the game, in the normal way. Happy to help!
From,
LuigisMansionDarkMoon
3DS Friend Code:1246-8984-3513