This trophy is for getting 100,000 Lucre total in one instance. While you could slave away for hours and hours fighting enemies, there is a much faster method that could net you the trophy near the beginning of the game.
First you need to unlock the forge by completing the "Watts Drops the Hammer" event. Afterward make sure you collect the following materials.
Wind Caps-Drops from Spiny Cones
Clear Feathers-Drops from Stinger Bugs
You can find both these enemies in the Luon Highway. After you get the items, go back to your forge. Choose the "Alter Armor" option and proceed to alter the same Wind Cap with four Clear Feathers. You might not think it, but this Cap will now sell for around 26,000 Lucre. So in total you would need 4 Wind Caps and 16 Clear Feathers to net you the 100,000 Lucre.
Once you sell them all you will end up with vast riches and an easy silver trophy!