When the leader of a group is taken out, their entire squad is also neutralized. When enemy leaders are within range, your troops will prioritize targeting them. They can be identified by their large flags. Your formation and the enemy's formation are important factors in deciding how a battle will turn out. Getting a better formation than the enemy is important. Try to use formations that avoid the majority of the group and flank the enemy leader. When you bubble an enemy leader, do not pop it immediately. If there are more groups approaching, wait until you bubble a new leader, then pop them all in succession. You will earn a combo bonus the leaders that are popped, which results in more medals.