When in the Arena, select players you think are easy to defeat. Although this rewards lower AP, it is an steady way to increase your total. That the better your ranking in the Arena, the better your daily rewards.
Coins
Complete the daily missions to earn coins. They can be spent to get items from the Portal or to upgrade your Heroes.
Elements
The element type of a hero will determine if that unit is stronger or weaker against a specific enemy. Tap the "I" button at the top right corner during a fight to determine their elemental status.
Hero enhancement
Enhance your heroes whenever possible. This will require various materials, which can be obtained from certain missions. Replay those missions, especially if they are early missions which are fast, to get the needed materials for your enhancement plans.
Hero placement
Your front line hero will attack first, as well as be the first ones to get attacked. Heroes at the rear are the last to attack. Consider keeping heroes with the largest amount health or most used in the front.
Leveling heroes
Level up your new heroes through potions to help them catch up. Do not use them on veteran heroes, which have been in use the start. Those veteran heroes naturally gain experience during missions. As you level your heroes, they will unlock more skills which also must be leveled individually.
Special attacks
Heroes have special attacks that will eventually become unlocked. The best time to use them is usually as soon as they are available. However, you may want to save some for special situations, such as upcoming Boss battles.
Special attacks
You can check your special attacks by tapping the "I" button at the top right corner during a fight. There are four different special attacks.
Attack Buff: Three elements of the same type; increases attack.
Defense Buff: Two elements of one type, one different; increases defense.
Dexterity Buff: Element 1, element 2, element 2; Increases DEX stat.
Recovery Buff: Element 1, element 2, element 1; increases the REC stat.
Arena mode...