Don't Use Too Many Strong Cards. Taking most or all of your powerful cards out into the questing field will bring your Mana count down even faster. It takes a lot of Mana to maintain strong cards, so you'll have a hard time during battles since you'll have to recharge most of the time.
Your Mana and Stamina will refill automatically once you level up. When you're not leveling up, your Mana will refill at a rate of one minute while your Stamina will refill once every three minutes.
Cards that are enhanced to a proper level pay off more when you make them a part of the evolving process. Make sure you enhance and evolve cards of the same Tier.
Coins can be acquired from quests as well as from drops provided by defeated enemies. Coins are used typically for upgrading your cards into more powerful ones.
Gems are used to summon powerful cards that you can add into your deck, as well as to purchase items that can refill your stamina and mana as well as allow you to fight in the Arena.
Summon stones and medals can also be acquired, with summon stones provided also by questing or via drops. Medals, however, are much harder to acquire. Both of these resources can also be used to purchase more heroes to add to your team, but their power is much lower as compared to when buying them using gems.