To avoid heavy setbacks rotate your troops at half dead. Siege, Field Battles, and sometimes Rebel Camps can be money drains because of the high probability of losing troops. Instead of having to farm supplies for new kits or spend all your bronze repairing soldiers, you'll be stockpiling it in droves. Battles like Expeditions, Rebels, Training Camps, and Free Battles all feature ways to minimize kit loss. How? The key to building bronze is not letting your troops die. I just bought the Monastic Knights along with a full set of kits using bronze alone and still find myself sitting on a 3 million bronze surplus. Many have quit the game altogether because of it without knowing my secret. I hear a lot of people talk about how they have no bronze.
This is one of the misconceptions this game has.