Two things players often neglect

If players only care about summoning purple heroes, or upgrading heroes, they may lose balance.

Players often forget to make use of Faction buff.

Different factions have their own powerful heroes. A lineup with identical factions combined together is better the one with all five different factions. Check the pic below:

Light faction is a universal faction, can be used to form a formation with heroes from any faction other than Dark. And Dark faction works by itself. Buff from Light or Dark factions is powerful.

The 2nd important thing: Faction Restraint

If you fail to pass any stage, try using Faction Restraint to change your lineup.

 

The additional 25% damage from Faction Restraint may help you turn the tide of battle.