You Are Not The Contents Of Your Wallet

If you're alone, the best is to keep doing Random Brawls. Easily 250ish coins each

If you want to check how much gold you collected towards the achievement, use this:

/run C_ContentTracking.StartTracking(2, 11567)

You will start tracking the achievement, and there you can see how much gold you have.

This achievement is still obtainable in this season's brawler's guild. The progress is account wide. You can either farm a single easy boss like Dippy with the [item=94162] for around 100 brawler's gold each win, or buy a [item=142319], and fight random bosses for 250 gold each win. Make sure to not overcap your gold, as you can only have max 5000 brawler's gold, so buy items as you approach the limit.