Other than awakening and leveling the heroes, the most importantly is also to level the hero skill level.
Skill level boost the hero skill damage and can be very deadly to the opponent.
Having one combat manual in the blue or prism slot can further extend the skill level beyond level 20.
Here a example of a hero which is commonly used by players.
Skill level from Level 20 to Level 24
(Additional 20% Skill Damage)