Despotic Arsene can be a troublesome superboss, simply for the fact that it has the highest Agility out of any enemy in the game. Combine this with the level difference (it is level 108), and you have an enemy that can be a little hard to hit. If you didn’t farm Night Vision crystals from Ancient Daedala, then you will want to do that before tackling Despotic Arsene (and the other two superbosses beyond this one). This is a reminder that Night Vision is for physical-based characters only, as it will not help Ether-based ones. You will also want to pile on all of the Agility for your characters, via Skill Links and Agility VI gems (they max out at 50). The best thing about fighting Despotic Arsene is that it has no Ether-based Arts, so everything will be physical.
You can find Despotic Arsene in Satorl Marsh, north of the Glowing Obelisk landmark. It will be mingling with the other Bunnias in the area, which will join in the fight with it, if close enough.
Emperor Blow - Physical-based Art that hits a single target for medium-high damage, while also inflicting Strength Down.
Imperial Sobat - Physical-based Art that hits in a frontal arc for high damage, which also causes Bleed and Knockback.
Imperial Upper - Physical-based Art that hits a single target for high damage, and can cause Blowdown, but not Daze.
Emperor Combo - Physical-based Art that hits two times for major/fatal damage, causing both Blowdown and Daze.
Imperial Might - Buff that gives Despotic Arsene Haste.
As already mentioned, physical-based characters will want Night Vision gems equipped on their weapons for this boss. Ether-based characters won’t have to worry about Night Vision gems, but should have the accuracy bonuses from Melia’s Enlightenment skill (Honesty tree) and Riki’s Bullseye! skill (Cowardice tree). These two skills combined should mean all Ether-based Arts will connect, no matter what. Of course, the problem with Ether-based characters, outside of Riki, is that they are a bit fragile.
The other bonus with fighting Despotic Arsene is that it’s surrounded by weak Bunnias, which is beneficial to you during Chain Attacks. Since you get a bar of the Party Gauge back upon defeating an enemy during Chain Attacks, you can use this to your advantage against Arsene and use area-of-effect attacks to try and defeat the lesser enemies. Other than that, the lesser Bunnias serve no purpose at all during the battle, as they won’t be able to hit you and Arsene gains no benefits.
Upon reaching 30% health, Despotic Arsene will activate an aura that increases its level by three. This will make its Agility a little higher (the maximum threshold for this is ten levels, by the way), meaning it might be a little harder to hit. As with any Aura, you can use Shulk’s Monado Purge or Melia’s Mind Blast to try and dispel it, but Arsene has a 70% resistance to those Arts. Naturally, for maximum proficiency (with the skills that involve nighttime linked to your characters), you want to start the battle at 19:00, especially with the Night Vision gems equipped.
It should be noted that Despotic Arsene only has a resistance to Break outside of Chain Attacks, meaning it is possible to Break and Topple it in normal combat. It is still better, if you choose to use that combo, to Break and Topple it during Chain Attacks, as it has no resistance to Break in those situations.