Darkja is heavy dark damage + instant death chance.
If darkja is killing all 3, it is just unlucky. If you are saying he uses it so often that it is a pain keeping everyone alive, then I agree.

(SPOILER)
After doing physical damage to him for a while and keeping my party members up and revived, he eventually turned on that physical barrier at 20%. I finally had to take out all my gambits except for ARise gambits, charge gambits, foe:reflect > dispel, and foe > shock. Otherwise my characters would spend too much time healing and buffing to attack.

He is a very nasty fish and you need to be able to worry less about buffs, and more about doing damage. His darkja will kill someone almost everytime and he casts it very often, so you can't be rebuffing protect, shell, bravery, faith, and bubble every time someone dies, or all you will be doing is buffing.