The best way to get builder huts its by completing the challenges. I believe there about 6000 gems you can get total from the achievements. While 80% will come at much higher levels, there is still a good amount to be earned! You can also remove tree, logs, mushrooms and plants for gems. If you just save you can get a good amount of them!
Some more advice, there are whats called goblin wars, when you go to attack look to the right for goblin wars. Even though it will seem dumn, for ever map you three star you will get gems, and once you get to the last 10 levels the loot will go from something small like 15,000 each loot to 100k, 200k, up to 800k! I didnt attack at on on these goblin wars because I wanted to save up for something big, so I did it all at once and saved up 7,000,000 elixer and got lvl 4 pekka! Play the game smart and it will work for you! And that is what im here to do, help you play smart!