10) You can get free gems by using your invite code to invite people to play and download the game. If you have multiple devices that can handle the game, then use your own invite code on a second device in order to get gems on your first device. The other of the invite code and the user of the invite code will both get 30 free gems for their trouble.
9) Another way to rack up free gems is to complete the various trophy quests. Rack up enough of these rewards and you will have enough gems for one of the gold chests, which earn you higher-tier armor than the silver chests do. Also, armor from the gold chest will power up your tower and your fairy too, not just your hero.
8) Go back to old stages and collect relics to not only increase your overall star score, but to upgrade your character, your tower and your fairy. They will provide a permanent upgrade to all of your stats, which will also increase the effectiveness of any armor and equipment that you buy.
7) The bosses get harder and harder, and later bosses, such as in levels 6 and 7, will immobilize you for long periods of time, making it very easy for their minions to open up your gate. Save up a bunch of ice storms and grim reaper power ups to immobilize them instead. Ice is especially effective as you can hit them rapidly after you freeze them, and they won’t be able to move at all.
6) When you get to tougher levels such as 7 and 8, the first 9 stages will be almost as hard as the boss battles on the previous one. Use one power up per stage, usually right at the beginning, and you will race through the stages rather easily on the way to the boss. Most of them will provide you a few less coins than just beating them with your sword, but the 2X damage power up will provide a TON of coins for using it.