You can win multiple bonuses at the end of a battle. There are bonuses for a fast victory, keeping all of your units alive, and finishing the battle with a special ability. If get all three bonuses, you will also have a 100% bonus to your gold reward. To get the time bonus, send your melee units after enemies. You cannot send your units to position themselves in the enemy portion of the battlefield, however you can send them out temporarily to engage any visible enemy. Do this preemptively to eliminate enemies as fast as possible. To get the special ability finish bonus, you must act quickly as it can be difficult to determine which enemy will be the last unit, especially there is not a Boss. Select the most likely last enemy unit, wait briefly to check if other enemies are appearing, then drop them down to low health and use a special ability to finish them.