You'll begin the fight alongside Pipin of the Steelheart (Gladiator), Alisae ("Red Mage" actually a Gladiator), Alphinaud (Arcanist with Obsidian Carbuncle) and Krile (Conjurer). Just inside you'll find two Laquearius, two Signifers, and a lone Secutor fighting some Stragglers. The Stragglers MUST be defended, if even a single one perishes you will lose the duty. Your allies will take the other four, so make sure you hit the Secutor a few times to get it off of the Straggler, then deal with them as you would any other monster. Your allies may very well finish theirs by the time you finish yours.
Past the river you'll find two more Signifers, a Laquerius, a Secutor, and two Magitek Reapers (And two Resistance Fighter allies, they are marauders). Your allies will take all but the Reapers, so as before grab their attention to keep the Stragglers safe, but take caution to also take them where their AoEs won't damage the NPCs.
In the markets you'll face a Laquerius and a Secutor, as well as a new beastman, the Ferae. Two bowwolves (Archer), a spearwolf (lancer) and a bladewolf (Samurai). Their attacks have different names, but mostly copy existing moves with the only one to really worry about being Iai from the Samurai, which mimics the conal Tenka Goken. You only need to grab the spear and blade wolves to do your part.
Lastly, you'll face Fordola, two Skulls Mages, a Skulls Blade and a Skulls Spear. Fordola is yours to fight while the rest are occupied by your allies. You CAN take out the others so your allies can assist you, but it's faster and likely safer to just focus on Fordola, who fights like a normal Gladiator. Just get her down to 10% for a cutscene.
Ok, true final boss time. You'll be facing Zenos Yae Galvus, alongside Pipin and Alisae. After a little while he'll use Vein Splitter, taking out your allies and dropping you to low HP (thankfully the story buff provides regen) and inflicting Stun and Down for the Count. Your goal here is to take Zenos down to 94% HP, but he's level 70 so it won't be easy.
During the fight he'll summon The Storm, a sword that does AoE around itself (Thunderous Force), as well as The Swell, which does AoE around the outer edge of the arena (Tempestuous Force). Zenos himself will use Concentrativity, an AoE around himself, and Lightless Spark, a conal AoE he'll often use alongside his summons to cover any safe spots. Occasionally he'll use a different version of Concentrativity that has the same animation as Vein Splitter, but simply does Concentrativity's damage albeit arena-wide.
Your main goal is to dodge all the AoEs while doing what damage you can. Surviving takes first priority. Zenos' melee strikes are pathetic and he walks slowly toward you if you run away, so as long as you can avoid the AoEs you'll be able to survive. The blade patterns begin easy, but as the fight drags on they will often combine to form undodgeable patterns, so break the swords to create your own safe spots.
Once at 94%, he'll use Vein Splitter once more, taking you out and ending the fight.
- Krile's dialogue after the fight differs if you are a healing class.