After saving Rosa on top of the Tower of Zot, You will face Barbaricca. This fight will turn into a nightmare if you simply attack with all you got. He will counter all attacks with a Lightning strike that hits all party members. This will knock your party out within a few rounds and your healer won't be able to keep up. Simply only attack when your parties health is high enough to take a lightning strike. This fight must be won by being defensive and take your time. The boss itself doesn't have that powerful attacks so you can take your time healing up before you hit the boss again.