The entire group that is hit sustains the reduction in defense. You can hover over the icon for any group that scrolls across the bottom of the battlefield and see the number of health points at any time. Those numbers (health and attack) are for the entire group. Even if no defense bonus spell is applied, there will be a reduction in health points and in attack points when any of their members are killed. If a spell is applied, there will be a further reduction for the rounds the spell is active.
If you were to hit one enemy group with a spell, get a defense reduction, check the number of health points and then wait however many rounds the reduction is active (it varies by the different units, sorceress is for 2 rounds, blossom mage for 3 rounds) you would see an increase in health points after the defense bonus expires if you had not hit the group again.
^^I've read and edited that 3 times and it's still as clear as mud no matter how I try to say it, lol! I'm leaving it as is and hope it makes better sense than I think it does...