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.
I works like this(If your in no random)
2)Lower number of stars
In even attack battles higher stars always lose and since leaders are all high stars. yeah they tend to lose
Whoever has the least stars wins an even attack battle (or if they have the same amount of stars, the one who went first wins).
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.
there is a FAQ section in the Game Rules. use it.