Under Siege

You will likely get this by simply playing thru the campaign. You earn gold as you kill enemies, and find treasure chests. Thou there is a hefty "upkeep" cost at the end of each map, you will find 5,000 easy to hit while within some maps.

Option 1: Save up gold when you start Chapter 1, only replacing the one archer that is killed before the rest of that group join you. Explore and get all chests, kill all enemies. Scrounge and you should have enough before the end of this chapter.

Option 2: On the 2nd map in chapter 3 (12. Deep in the Forest - where you are asked to collect treasure), I personally collected 5915 gold. That's not including the gold I started with, and I likely missed some treasure and enemies too.

Option 3: On the 2nd map in Chapter 5 (19. Mech Control), I personally collected 10345 gold. That's not including the gold I started with, and I likely missed some treasure and enemies too. See Chapter 5 for some info on treasure chests.