What is the fastest way to get money?

Early on the game, the best technique is to fight the highest level monsters that you have access to.

Here are some more techniques:

• Gold Golems, which you can find in the ditch south of Gleeba, drop 505 gold each.
• Use Alchemy to make Ear Cosys (Fur Hood + Bunny Tail + Lambswool) and sell them. You can buy the ingredients at Slurry Quay (Fur Hood), Porth Llaffan (Bunny Tail) and Batsureg (Bunny Tail).
• Use Alchemy to make Gold Mails (Silver Mail + Gold Ring + Gold Bracer) and sell them. You can buy the ingredients at Bloomingdale (Silver Mail) and Porth Llaffan (Gold Ring and Gold Bracer).
• In mid level (Copper, Bronze, Steel, Silver, and Gold) Cave and Ruin grottoes, you might find Gem Jamborees - these are worth 1130 gold each, and drop Gleeban Gold Pieces or Gold Bars.
• In high level (Platinum, Ruby, Emerald, Sapphire or Diamond) Ice (Snowhall, Tundra, Glacier) and Ruin (Crypt, Ruins, Path, Maze, Lair, World) grottoes, you might find Gem Slimes - these are worth 10080 gold each.

(Most of these techniques require finishing certain points of the game.)

I choose to make ear cosies. Killing gold golems are more fun but is not very efficient. I am pretty sure making gooey gear is most efficient. To get a lot of slimedrops I go to the plateau by Angel Falls and kill slimes.