Contrary to popular belief: There will not be an endless spawn of enemies. Every 4 turn 2 enemies spawn, and every 8(or is it 12?) a bunch of enemies come. There are 3 of the big groups. You can easily turn 4 times, kill 2 enemies, turn 4 times, kill 2 enemies, turn some more, take care of group and repeat it.
- Each group seem to contain a mix of ranged, melee and tank mobs. The priority with the red templar are the Knights that turn other red templars into abominations. Have Cassandra get aggro of the bulk, and try to get rid of any archers/ranged as quickly as possible.
- The last guy, the behemoth, is easy enough. He can be frozen. This means as long as cassandra gets aggro of him, you can get rid of all the smaller enemies and you can CC him. Even if he does kill somebody, you can freeze him and proceed to resurrect your dead party.
For builds:
Your tank: Build for all the passives in the sword and shield tree. 20% damage reduction is huge! 50% less damage from ranged at front? huge! Then get both taunts (War cry and challenge) in Vanguard. Upgrade War cry. You should also get the 20% armor passive in vanguard. Then end it with Grappling hook and possibly Combat roll in Battlemaster.
Your mage: Build heavy into spirit for barrier. The resurrect is pretty nice to have! I have had times where my whole team but solas is dead, at the same spot, and he get them all up in a few second! Also, get Winter's grasp. Important CC!
Your ranged dps: Get Stealth, then get the whole right tree of archery. I also get posion + its upgrade. Stealth is important, obviously!
I do like to get the end-ability of the archery tree, just so I can quickly go over to Varric, target an archer/mage, use it and go back to my inquisitor. That ranger/mage is dead once Varric is done shooting.
Yourself: Do whatever.