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.