First thing I'll say - Save your gems! If you are planning on playing this game and not spending a dime or very little - don't use your gems to speed up the opening of chests.
You start the game with 100 gems given to you, if you get in the habit of opening chest with these you'll soon run out - Really they are better spent being converted to coins which can be used to upgrade your cards later on. And believe me, once you get to the higher levels you'll need a lot of coins to upgrade your coins. However you do earn more (coins and gems) as you progress through the game.
After you have been through the training matches you'll then be thrown into real bouts in the Arenas!
The trainer battles are pretty simple, but if you are having trouble here are a couple of tips below...
Basically wait until you have full elixir (the purple bar at the bottom of the screen gets to 10) then basically you want to so the long range cards (archers, musketeer or skeleton bomb man) behind your stronger cards (giant, knight or prince). These cards will give protection to your long range cards and you should be able to pick off these trainer towers pretty easily.
We'll start our guide at this point with recommendations on how to play your cards and which ones work well together.