Deepvoid Slave.

[Krausus]

Uhn... I'll add this here given my experiences: Deepvoid Slave, like any other boss battle in this game, requires having a good synch with your party regardless.
My LS took like 8~ or so tries (with 3 out of our 8 having done that fight and won it previously, also a handful of us were derping there) until we realized that actually attacking that sucker from the back IS a good idea, if used properly. The thing is, if you attack it from the back it'll do the elbow drop, which is a conal AoE, and that takes priority over all of it's other moves, so you can essentially keep it busy elbow-dropping all the time and prevent it's AoE skill and prevent it from causing too much damage to the tank.

its not a bad strat as long as your melee know what they are doing as well as the tank. If you happen to need to move it then the tank cant get to far away or he will spit on everyone and that can easily take out a few members.