Barbatos has a total of three attacks: a standard physical attack that inflicts comparatively major damage, Body Splash that hits the entire party for physical damage, and Mortar, an even stronger attack that affects the entire party.
Barbatos has high Defense and the highest HP of any regular enemy. It is difficult to inflict Overkill on Barbatos, because of its large Overkill HP range and tough Defense.
There are several ways to defeat Barbatos. The safest way is to cast Demi (preferably using Doublecast) to bring Barbatos's HP down to a manageable level, before finishing it off with conventional attacks.
If the player has a weapon with Zombiestrike, they can use it to inflict Zombie on Barbatos, and then cast Curaga on it for several thousand points of damage per casting. Barbatos cannot, however, be destroyed by revivification abilities.
If the player can withstand the threat of Mortar, they can use Armor Break to destroy Barbatos's defenses, but this will only work 50% of the time. The player must ensure all characters' HP are topped up as Barbatos will use Mortar capriciously.