Control
You have the option to either auto-battle or manual battle and although one is not more effective than the other you will find that the best strategy is to set the game on auto-battle and take manual control of spells and special moves.
Story Mode
The most lucrative rewards which includes hero EXP and Crystals can be earned during Story mode where you will be rewarede 320 Cyrystals for completing it.
Clear Dungeons
Try to clear Dungeons when you enter them as they are are a rich source of gold and will help you level up your equipment quicker. You will find that dungeons will become more difficulut as you progress so take advantage of the easier ones.
Spending Crystals
Don't wast Crystals on Chests, it is more beneficial to spend 100 Crystals to enter the Daily Dungeon a second time on the days that reward you with Armour, Weapon, and Accessory Heavenstone.
Reset Daily Dungeon
You can spend 100 Crystals and reset the Daily Dungeon which will allow you to complete the dungeon one more time at any level.
The best place to farm for Heavenstones is the Arena.