to beat the thunder dragon just use strong units such black knights and yettis to stop him. then use ranged units to attack him. Make sure to clear all units so you are fighting him alone( almost alone anyway) comet helps alot too
I Finally beat the last level on my third try. What did i do differently??? - I didn't play.... lol .
When i played the A.I. used a massive counter attack right before I destroyed his castle. When I left the game alone, only a fraction of my troops would attack the castle and therefore no more overpowered counter attack!!! yay!!