To get more gold in-game,you keep the battle(siege mode)longer with weaker units. So,use

Swordsman(do NOT use very good killing units etc.master swordsman)


Archers(never use HUMONGOUS groups of them)

That's for act 1 ,2 AND 3

act 4:gunner(lv 1-3)

act 5:vultures Lv.1-Lv3 and wizard lv.1

act 6:Wizard lv 1-2

OAR lv 1,2 and3

HAR lv 1