Quickest answer:  Use strategy.

Short answer:  Win more dungeons, unlock more Quest Rewards, Unlock Free & Boss Chests, Sell Hero Cards for Gold, Level Up and Evolve Heroes, use Skill Scrolls, bind Runes to your Heroes, use Potions during Battles, use Gems to revive your Heroes when you die in Dungeons, Add Friends, Join a Guild.. 

Long answer:  Oh dear! Dungeons can be a dangerous place. Your progress there may be hindered by not having enough health or by trying to battle a Dungeon that requires more Party Power than your current Party has.
Fear not, however. There are many ways to progress in Battlejack.
It is a safe bet to re-enter a Dungeon you have already won and get more Experience, Boss Chests, Friends, Hero Cards there.
You can also wait for the Free Chests to unlock. The contents of the Chests can be used or sold. There's more about the possibilities of the Chests below.
Think colors! If you're having a hard time because you're being burnt to a cinder in the Desert (Chapter 3) - bring some Water to extinguish those fires. If those Pegasi and Unicorns are getting you down, pack some Dark into your Team and start wrecking those Temples. Light Heroes can be used to battle better in Dark dungeons..
Leader Skills can have a huge impact on how the dungeon plays, as does picking the right Reinforcement.
Some monsters have very high defense so you might not get much damage through to them. Use piercing runes to get through their defense, a skill like burn or poison, or just go for that 21 for massive damage.
Some monsters have a skill that flips your recovery stat to negative so you can't heal at all! It may be smart to not use healers and use other potions like star potions instead of healing potions against such boss.
Some monsters and Titans have a skill that can flip your DEF to negative, making your shield zero! When the monster flips your DEF to negative, it will deal more damage the more DEF you had, so using heroes with strong health and attack can be more effective than heroes with lots of DEF or DEF runes.
So, pay attention to what skills the monsters have and what they can do to you, and try to come up with a strategy to counter it.
What you can get from completing Dungeons
  • Experience - Increases Player Rank
  • Gold - Can be used for Leveling Up and for Evolving your Heroes, Upgrading Runes
  • Potions - You can use these in future Battles to survive better
  • Fulfilling quests - Unlock Rewards such as Gems, Gold, Honor, Honor Summons, Rare Summons and Hero Cards
  • Friends - Whenever your new friends borrow your Heroes, you get Honor which you can use for Honor Summons
The Experience increases your Player Rank, which in turn unlocks more features, increases the Power, the Maximum Size and the Maximum Energy of your Party and Quests Rewards.
What you can get from the Boss Chests and Free Chests
A Chest may contain any of the following: Gold, Skill Scrolls, Potions, Hero Cards, Honor, Rare Summons, and Runes.
Skill Scrolls can be used to increase the Skills of your Heroes, increasing the efectiveness of your spells.
Potions can be used during battle to cure your team, increase their Skill levels, and to cure the team of negative statuses (a Hero might be Frozen, Cursed, Silenced, Burning, Poisoned etc).
Hero Cards are placed into your Hero Collection so that you can Evolve the Heroes you like, increasing their health and power.

The Honor and Rare Summons add Hero Cards to your Hero Collection.