I guess the best strategy in this game is defense & counter: upgrade your defense cards with delay +defense, then get counter. Rack up your defense and use counter when the enemies ready the big attacks.
The key to win is to have 5-6 monks, then your protection spell could last long enough for one wave. Keep only 4 melee at the front row, put on a few bombers, then the rest archers. Use protection at wave 1/3/5, and the rest at wave 2/4.
For those who wondered how to "complete the game", after completing the fourth heartstone quest, go to your side's camp near the fourth cave, you will receive a new quest to beat the final boss. Kill the boss to complete the game.
If you want all the accuracy stars from normal levels, a good way is to max secondary weapon buffs and molotovs. Two fully upgraded molotovs should cover the entire field and kill most enemies. Shoot only those that occasionally get past the fire wall to get a good accuracy rating.
That depends on your internet connection..!