|You'll first face a Laquearius and Signifier in the open world, a marauder and thaumaturge respectively. Take out the thaumaturge first before beating on the tank, then meet Raubahn for the real fight.
You'll be aided by Raubahn, Alisaie, a Flame Pugilist, and a Serpent Conjurer. Throughout the fight Raubahn will be fighting Grynewaht (Uses Augmented Suffering and Firebomb for AoE damage centered on himself and a random target respectively, and Augmented Uprising for conal damage), the pugilist will fight the Hastatus (A gladiator), the Conjurer will be fighting the Princeps (A thaumaturge), and Alisaie will be fighting the Prototype. Of these, only the prototype needs to be defeated, the three others have a prohibitive quantity of HP and defeating them yields little benefit. Both the Conjurer and Alisaie will support the party with healing.
Focus on the Prototype while dealing with the adds, which usually go down easily enough to AoE. At first there will just be two slasher adds, but more will be summoned as the fight progresses. In this first phase it will use Cermet Pile, a line AoE, at whatever it's targeting. Once taken below 90% four more slashers will appear and the Prototype will begin using Open Fire, which hits several large random circles on the battlefield. At about 75% it will summon two Hoplomachuses and two Signifiers.
Finally, at 65% it will begin using Mine Layer. This summons a Magitek Turret that will tether to you, and slowly fly to you until it self destructs. Simply lead it away from your comrades until it explodes. At 55% it will summon about 7 more Magitek Slashers, and Raubahn will ready Immortal Flame. Just survive until it goes off and they will be defeated. At 40% a cutscene will play and the final phase will begin.
In the final phase you'll be joined by three Resistance Fighters (A gladiator, lancer, and archer), M'naago of the Crooked Fletch (archer), and Conrad Farsight (thaumaturge). You'll need the extra DPS as the prototype will begin channeling Self-destruct, and if it goes off you wipe. Just focus all the damage you can, you have 1 minute to take it from 40% to dead.