The entry area right before the 2nd boss at the end of the Dark Forest, is filled with pots. Stomp on the 6 small pots, leave, come back. You get a 1000 gold bar drop every 2-3 rotations, and a rotation takes about 5 seconds, about 5000 gold a minute. Grind as much gold as you need for achievements and upgrades.
Gold-related achievement progress registers when you save the game, quit the game, and re-load your save. Do so to prompt this and other Collect X gold achievements to unlock.