When I fought the Cloud of Darkness my party was Ninja (back row), Black Belt (front row), Devout (back row), and Summoner (back row). My first turn I had my Ninja throw a shuriken, my Black Belt boost, my Devout hasted herself and my Summoner casted Bahamut. From then on you kind of have to take it on a turn-to-turn basis. My party was level 58-59ish and my Black Belt had the highest job level of 61. They all had ribbons equipped as well. My Devout had 14 casts of curaja available to her so every turn I casted one of those on the entire party and depending on the situation, I would have my Ninja shuriken and my Summoner cast Bahamut again and my Black Belt would use a phoenix down to resurrect my Ninja if the particle beam took him out in conjunction with the other physical attacks he took. A few times I thought it was over but I pulled through in the end with everyone alive. Just make sure you can cast a curaja every turn or have two Devouts so one can cast curaja and one can arise the dead.