Alright, here's what you do.
If going Warrior:
Roll as two-handed Warrior.
You can avoid the majority of mobs in the Prologue Area by skirting the map. Do it. You don't get XP from them so there's no point in doing it.
Take the Charge Option.
When you face off against the Pride Demon, disable Cass' taunt. Let him pound on the Soldiers, you do more damage disrupting the Rift than actually attacking him, so have your team focus on the adds, not the Boss.
When you get free, respec yourself and Cass.
Grab Mighty Strike and Chain. Give Cass Upgraded Payback Strike, and the SNS Block ability. Your job for now is to reel in ranged targets.
Get Sera, or Vivienne ASAP.
Respec Varric at level 4. Get Toxic Cloud and Lost in the Mists asap. Now you are tanky enough to do as you please.
As a Rogue
See my Rogue post in Matth's Pre-Skyhold builds.
As a Mage
Barrier, carry a Staff for at least two elements and scout ahead with Tactical View.
I'm on my seconds play through with All Trials on, and its enjoyably challenging!
Basically, you have to rely on other forms of healing (Regen and Healing Mist items DO NOT WORK PERIOD). Poisoned Weapons for Rogues, Good Guard maintaince for Warriors, Barrier uptime for Mages.