Wow, managed to collect a score of 9 on this thing :)
I like it, quite some strategy involved, likely also affected by your ability to time your hits on the rolls (I am really poor at this!).
For me what I found working is roughly as follows: Start with merchants to fill the slots (less chance to roll too bad with dragons), then preferably 3 coin slots to allow for buying the better stuff. Do this until being able to get a few swords. Then slip in a few wizards (when not enough coins to get a sword) to allow for cleaning up a bit. Towards the end pay attention to cleaned reels to see where swords are the most needed (on the reel with the most dragons, or later the last dragon, swords are not useful).
I didn't find metal and furnace too effective, likely due to my lousy skill at timing my hits. Otherwise it seems to work best if focusing on one reel for furnaces, preferably grouped tight so to make it easier to hit them, and the other three reels for metal. The rightmost reel if you would like to try to get the furnace if you happened to roll metals, the leftmost if you prefer to try picking furnace first and try to gather metal with the further rolls.
Quite fun little game!