Nice and simple way to increase your profit is to move your mines around. For example, moving your level 4 mine from area 1 to area 10 will increase your earnings. Resources usually get better with depth.

With new area and new bosses comes new spells as well. Try them out. More often than not new spells work better against bosses that follows.
Be sure to upgrade spells as much as you can.

Keep an eye on the Quests they can get you in serious trouble if you don't have what it takes to finish a Quest.

Finally, upgrade buildings and learn new recipes for even more resources.