Just to clarify, you attack the defense leader, and if you win the fight, the defense leader goes down which allows you to attack any of the members of the opposing syndicate. I believe it's typically better to power attack a defense leader (uses 100HP) if you're going to lose anyway, although the power attack gives your attack stat a 50% boost (e.g. 30k atk --> 45k atk) since the power attack will take down half of the DL's health, as opposed to four separate normal attacks (since a loss to the DL removes only 1/10th of the DL's health).
I could be wrong though-- I've heard that if the DL has a defense way, way stronger than your attack, it takes 3 power attacks to kill the DL instead of two, but I can't confirm this since it's never happened to me.