Crypt of the NecroDancer

I found a really great seed for this achievement! Choose Bard as your character, then go to All Zones Mode (seeded) and enter: GOLDEN

Make sure it's in all caps. Here is a breakdown of the items you can (and should) get. Since you have unlimited time with Bard, kill all the enemies for maximum gold. Take your time and play it safe so you don't lose your multiplier.

1-1 Get the Golden Rapier as soon as you can from the blood shop. This will increase your gold on each kill. The barrel contains cheese, so you might want to grab that too.
1-2 You can get a heart container in whatever shop is from the travel rune (Health Shop?). There are 4 bombs in this level, so pick those up. Buy the Bag of Holding from the shop, and get Obsidian Armor from the barrel (push it down a trapdoor if you don't want to waste a bomb).
1-3 Bomb the shrine to get a bright torch. Buy a monocle in the shop. Kill the mimic chest and get the Titanium Crossbow -- this will lower your gold compared to the rapier, but it's a much better weapon, so for me it was worth it. I still ended the run with plenty of gold.

Kill KING CONGA flawlessly to get a pickaxe.

2-1 Ring of Gold! This will get you more gold, and you won't have to manually pick it up anymore.
2-2 Holy Water
2-3 The Conjurer shop is in this level, if you want a random item. There is an exploding mushroom in the main shop, so shoot it to blow up the gold walls for extra gold.

3-1 Use the Shrine of War to cause all monsters in future levels to spawn as their hardest versions, netting you more gold. Your crossbow should make it easy. I would keep the monocle if it gives you a Circlet of Telepathy. There is a transmog shop here if you want.
3-2 Do NOT use the glass shrine.
3-3 Nothing special.

Kill DEEP BLUES flawlessly and get a Riches Scroll (be sure to use it while you have max multiplier)

4-1 At this point I had $4k in gold. There is a heart container and Boots of Strength here.
4-2 Blood Shovel -- use this to dig the golden walls of the shop, if you want, but it takes away health. I think I used it a little bit, healed, then swapped back to my pickaxe before leaving this level.
4-3 I had $5766 gold coming into this level, and I quickly got to $6k and the achievement popped! If you have extra bombs, you can use them on the golden walls of the shop. I finished the whole run with $7114, so you should have plenty of leeway to buy some extra items along the way if you need to. It's also possible to get a much higher score, but I was playing it safe and just wanted the achievement.

GENERAL STRATEGY IF YOU DON'T USE MY SEED GUIDE:

There are a few items that can help. Ring of Gold, obviously, and you can also kill Freddie Merchantry (the Shopkeeper) and take his Crown of Greed. This will double your gold, but every step loses 1 gold. If there is another crown on that level, swap it and kill all the enemies, but don't pick up the gold. Now swap back to the Crown of Greed and get all the gold.

If you can get a Ring of Wonder, that adds 2 gold to every enemy drop.

You can bomb the gold walls around the shopkeeper for extra gold, or dig them up with the blood shovel (if you have enough health). This is probably the best strategy (Crown of Greed + Blood Shovel).

Here is a video of Spooty getting $20k in one run, using Melody, blood shovel, crown of greed, ring of wonder, heal spell: