Alternis uses attacks befitting his status as keeper of the Dark Knightasterisk. By using Dark Bane, he sacrifices some of his HP to attack a single character, and also uses Black Bane—which damages all party members instead of one.
After he loses about half of his health, he starts using Minus Strike, which deals the number of damage equal to the amount of HP he's lost. This is guaranteed to kill a character in one hit. It is recommended not to use the Black Mage's damage dispersion, since slicing 9999 damage (the maximum damage one party member can take) on every character would most likely still kill the targetted team member with 6500 damage or more, as well as dealing heavy damage to each of the other alive party member (near 1500 damage), potentially killing them.
Spiritmasters with the skill Enigma is recommended to protect the party from Dark Bane and Black Bane. Templars with the move Rampart are also recommended, as they can shield the party from Minus Strike.