Here's something interesting I found out last night: Reyn is a better tank than Dunban for Despotic Arsene. Dunban is still great and all, but the fact that he always dies against Arsene's talent art + Imperial Sobat/Emperor Blow/Imperial Upper combo was starting to annoy me. Plus, he struggles to hold aggro from 7th in the Arsene fight. Last night I went with a full physical tank build: I linked Shulk's -5 weight skill and equipped Dunban with heavy armor (M100 & Ledios) with no agility penalty. I also linked 7th's 10% less phys damage skill, for extra tankiness. And still, Arsene's combo killed him — even with M. Armor up. I looked it up on the Xenoblade Wikia, and it says this:
Emperor Combo X - Despotic Arsene's Talent Art. A severely damaging 2-hit combo. Almost impossible for Reyn to survive, even with Last Stand active while he is under the effect of Sharla's Shield Bullet (unless he guards with Guard Shift to half the damage, or uses Shulk's Shield or Armour talent arts).
So I decided to use Reyn, figuring I'd be able to warn him to use his Guard Shift art; combined with Armor he was sure to survive.
I equipped Reyn with the same heavy armor set, Dunban's 15% less phys damage skill, 7th's 10% less phys damage skill, as well as Shulk and Sharla's +5% defense skills. I didn't even need to use Guard Shift. It was ridiculous; Reyn was tanking every move from Arsene with ease. The dreaded combo did a good chunk of damage to him, but a Light Heal + Aura Heal VI gem had him back to full HP in no time. And with Night Vision + 7th's Agility Down gem, he had no problem hitting Arsene and holding aggro at all. I tried this three times and each time we kicked Arsene's ass without any problems.
Dunban is still a better tank against the other 3 superbosses, though.