It really isn't that hard. On my first run through the game, I had a Knight, Dark Knight, Devout and Sage. The Devout has to cast Curaja every turn. The Sage shuld be filled up with the most important white magic: the 4 cures, the two raise spells and haste. Fille the other slots up with Black Magic. That's all you need, IMHO. Level 55 and higher should make this easy.

During the battle, as said before, Curaja every turn with the Devout. I used my sage to haste my Knight and Dark Knight. After that, he either aided in Curaja'ing, or used Flare or other high level spells. Do not bother to go after the tentacles, just focus on the Cloud of Darkness herself.

A similar strategy should work for you. Haste your fighters. Curaja every turn. If you have shurikens, move your Ninja to the back row and throw Shurikens on the CLoud. Have your Dragoon either jump or use attack, it's your choice. A good idea is to give your Dragoon one Blood Lance, so he'll recover HP with every attack. Again, don't go after the tentacles, just focus on the Cloud of Darkness. Be lv 55 or higher, and it should be a cinch.