You would need to provide more information. Such as: Dagger or Bow Rogue? Skills? What spells does Dorian has? I imagine Cassandra is a Tank and Iron Bull a Reaver?
You could always load the Milestone, even if you can't load your own saves from prior.
What consumables do you have (grenades/tonics/etc.)? Would also be valuable before we can help.
Well, I'm a double dagger rogue, and I don't use any tonics or grenades, just the healing and health restoration potion.
I also have all the double dagger skills and some stealth skills such as Stealth, Easy to Miss, Evasion and Evade.
Dorian is a fire mage, but I`m currently using a lightning staff since the Nightmare has a weak spot on that. He has the barrier ability, Fire Mine, Immolate and some Necromancer skills that aren't useful at all for this boss.
Cassandra is a weapon and shield with all her abilities on this skill tree upgraded, I also have some Vanguard skills and I look forward to get the Spell Purge, which would really help a lot.
Iron Bull is indeed a Reaver, and he uses a two-handed Hayder's Razor. The only problem with him is that usually he is the first to die, since I start the battle with only 4 potions..
Thank you so much, if you guys need anything else just ask.