VP2: Finished Chapter 3. Fucking finally! It only took nigh onto 7 years to do so. Also the weapons I got from my departing party members are fucking sweet.
Who are you bringing to the fight and at what level?
Additionally, have you tried equipping some Terrain Defense gems on your allies?
Shulk, Reyn and ADMIN EDIT: Seventh.
I've got Poison on their weapons as well as Topple and Daze Defense and it doesn't seem to make a bit of difference. ]
I see what you're doing wrong here.
First off, get rid of Shulk or Seven. Seven is a powerhouse but lacks in durability even when specced in speed and Shulk's piddly little Light Heal ain't gonna cut it for her. I'd recommend benching Shulk for now and going with either Dunban, Melia, or Sharla (in your case, probably the latter of the three).
Second, ditch the Poison gems. Unless your name is either Riki or Melia, you have pretty much no business with Poison. Bleed is a much better status effect to invest in as you have more people capable of inflicting it. Moreover, it might behoove you to invest in an Agility Down or two and a Strength Down or Ether Down gem, you'll take less damage from the resulting attacks (and I recall this boss being pretty Ether heavy) and more importantly you'll take less damage and deal more with an Agility Down proc (seriously, with a loadout like that, I took a party of Reyn, Dunban, and Riki through the majority of the postgame content). Also, if you don't have them already, get as much Agility on your guys as possible. +50 is the highest you can go via gems and it should be easily doable at this point of the game with two gems apiece (even if you have to go to less impressive armor for their embedded Agility V gems).
And finally, if you're having trouble keeping the fight in the center of the arena, then you might want to consider playing as Reyn for the fight so as to keep the aggro and thus the fight away from those corners. There's another gem called Good Footing that you might want to consider looking into as well as Aura Heal and Debuff Resist gems to avoid taking Topple and Daze.