This game is relatively new since it was just released on 14 march and therefore there isn't much content. It is really similar to summoners war making it a train your monsters and watch them fight game here are somethings i wish i knew when i just started. Edit
Save your complete missions exp and use for when you are just about to level up
Use Gold to buy 1 stars for exp ( stones and 2 stars are a waste, the equips in the shops are no good.)
Use Gold to upgrade your equipment (1-3 stars don't spend so much gold on it)
Spend your trophies to upgrade your towers > speed keys > skelgon (in order of importance)
Reroll for four star heroes (succubus and fourhands are very good) even better for five stars if you have time.
Build your team around to make use of leader attributes
spend rubies (900 package) till you get good heroes and after that spend it to refuel your keys
don't use three stars and higher for merging.