The best way to make gold is to create a crucible level with 9 waves of 100 skeletons and give yourself as many negative effects as possible (No HUD, lower mana regen, etc). You can make ~10,000 per match which takes about an hour. You will also gain gold throughout the game, likely somewhere near 30,000 in the two required plays to get all the other trophies.

If you want a change of pace you can play online with someone else and use their characters and still retain the gold you collect. By far I found the crucible method the easiest yet one of the more boring.