Cause as much damage to the remaining enemy heroes before destroying your opponent's fort. Try to fill the Arena's damage reward as fast as possible.
Attacking
Focus on the enemy fort first, then the heroes. Select the proper order when your heroes. Soften up an enemy before finishing it off with a more powerful attack from a different hero. Begin taking out enemy heroes with your front line, then attack the fort with the rear line. Reposition them to get the bonuses. Have a full row of heroes deployed to attack the fort easier and destroy enemy heroes. Do not allow your opponent to have larger numbers. Limit their heroes to two or three heroes per turn. The more heroes you have active, the more damage you will deal. If you fill the board, swap lower power heroes with better ones.
Hero special Skills
Some heroes have special skills available. For example, Alex has Rush which allows him to attack immediately. Mitch has Nimble, allowing movement to any square and gives him 10 power each time a friendly fire hero is deployed.
Items
You can bring a limited amount of items into battle. Choose ones that favor your play style and affect my heroes or enemy heroes. Ignore those that affect the forts. Use them at the correct moment for best results.