His Leader ability says all your melee attacks can attack the flank and if there is no flank heroes they can attack the rival leader. Does it mean:
A - Only YOUR leader melee attacks
B - All of your heroes melee attacks in addition to your leader?
B. You can only Melee attack the opponent's Leader (with your Heroes and Leader) if your opponent doesn't have any living Flank Heroes.
But you can always target your opponent's Flank heroes with Melee attacks even when they are not in Melee.
Hope that helps.