Oops, sorry, I misread your post,dwnforce.
I just played through the Jarvia sequence at level 14 and it was still really tough (this is my fourth time through the game). The best party for this fight is heavy on mages. In mine I have Morrigan almost all the way through the Entropy tree so she can paralyze groups of people without harming party members, and/or put them to sleep, so your fighter(s) can take out one or two enemies at a time. Of course the top end of the room is heavily trapped, so you want to keep your fighters or assasins from chasing up the stairs. I also had Wynne using glyphs and Paralyze/Stone Fist and buffs, and of course healing all the time, with my assassin backstabbing with poisoned blades and disappearing in and out of the fight. The fighter du jour - in this case Alistair -- just kept whaling away at the weaker enemies while the mages kept Jarvia herself nicely locked up with freezing, paralysis, sleep and horror so she can't do too much damage, and whenever she disappeared, Morrigan shot another group level spell in her general direction. I don't do well in this battle without my two mages; the fighters tend to go down pretty quickly. But that's probably because I don't seem to build good fighters. I usually play a rogue/mage game, with heavily built rogue/assassin types. Fighters often end up as sacrificial lambs in my games.
Hope that helps.