What's the mistake to avoid at the Center position?

The classic mistake: treating the Center as just a third damage dragon, or putting anyone there. The Center spreads its aura to the whole army — it's the position that multiplies your other two dragons.

Typical errors:

• Putting a hitter with no useful aura at the Center instead of a Champion that gives the whole team -27.5% damage taken.
• Forgetting to match the Center to the fight: an endurance aura ("rounds 7-10") in a mini-fight that ends in 2 rounds means the aura never triggers.
• Placing a red dragon there for the type you field, sabotaging its own aura through reduced stats.

The right reflex: choose the Center for what it gives the OTHERS. Champion by default (damage reduction), Warrior if you want Initiative, big burst only in mini-fights.