Finally killed Jarvia (leader of Carta) on nightmare after like 2 hours -.- ! Was a lot of chance and lucky spells thats she didn't resist but I still feel like a bad ass.
P.S. I Did it at lvl 9 :)
Using: MC (Warrior tank), Leliana (Archer), Dog (Doge), and Morrigan (had only unlocked Heal and Force Field)