Monster World IV

Missable

Apparently you cannot unlock this once you've finished Tower of Silence (unconfirmed).

Simply pick up the coins dropped by defeated enemies, You need to get exactly 777 gold coins, then "something happens" (contained in spoiler), apart from getting a trophy . I got this during Tower of Silence, however you can farm most enemies by leaving and revisiting a screen.

Thanks to majinreid for the following tip:
Maybe it's obvious, but if you manage to get xx77 gold, you can always just go back to town and sell and buy your shield or armor over and over again until you get down to 777 gold.

Just remember to save first so you can just reload back into your area once you've got the trophy.

Your gold is increased to 7777