If the attack of the two cards are the same, then the winner is the card with fewer stars, if the stars are even then the attacker wins. So yes leaders will almost always lose even attack battles since they are 5 stars.
In fact the only even attack battle they will win is if the defender is also a 5 star.
In the event of equal attacks in a non random room, the player controlling the lower level character wins the round. If both the characters are at the same level, then the player controlling the attacking character wins.