I was level 10, character was setup like this:
Str 13
Dex 16
Will 16
Mag 50
Cun 14
Con 12
Torch of Embers
Helm of Honnleath, Tevinter Mage Robe, Enchanter's Footing (Rough)
Buckle of the Wind's, Apprentice's Amulet, Ring of Study, Iced Band
Armor 6
Defense 55
Spellpower 51
Spells:
Arcane Bolt
Combat Magic (only used passively for Helm of Honnleath, may not have been great idea)
Winter's Grasp -> Frost Weapons -> Cone of Cold
Flame Blast -> Flame Weapons -> Fireball
Walking Bomb -> Death Syphon -> Virulent Walking Bomb
Glyph of Repulsion -> Glyph of Warding -> Glyph of Paralysis
Mind Blast
Basically every time I tried this, I would be hit with Holy Smite right off the bat, and stunned for a long time before I could even use a spell myself. The exception was that somehow Cone of Cold could be cast, even though it looked like I wouldn't get through the animation, so I could at least briefly CC the Templar and Desire Demon. I also decided to try to place Glyph of Repulsion such that I would be standing on it while stunned after the cutscene, and getting the positioning right on that was tedious. I had to take the immediate "Die" dialogue to force the confrontation or the glyph would time out. This generally protected me from the melee attacks while stunned, so in theory I could survive the Holy Smite stun.
The problem was the Desire Demon liked to cast either or both of Cursed Dance and Horror before the stun would wear-off so I would just go into another CC state, and then the glyph would expire or be dispelled, and I would die. She would also mix in Winter's Grasp, which had a limited duration, but added to damage.
So reload, then reload. And so I started off with lesser ice salve, then placed the glyph, ran in, chose the "die!" dialogue, paused before the music could start, Cone of Cold... and this time I was surprised to see that I wasn't stunned. Either I froze the Templar before he could cast Holy Smite or this was the one time in 100 that he didn't open with it immediately. So then it was Fireball to CC nearly everything, retreat out of the room and then Glyph of Paralysis for Paralysis Explosion, Walking Bomb and Virulent Walking Bomb on Desire Demon, and then worked on two skeletons that weren't CC'd. The Desire Demon was free by then, I retreated further, another Fireball to finish the skeletons and the demon, at which point I noticed Leliana's portrait was gone, and Wynne was starting to take damage. Apparently a random skeleton found the bad spot I tried to hide Leliana and Wynne on hold and they were tanking. So the lucky break at the beginning was wasted since tanking teammates meant another reload.
Started off the some way again: lesser ice salve, glyph of repulsion, "die demon," pause, cone of cold, stunned by Holy Smite. Glyph was in the right spot. Leliana and Wynne moved to different spot on hold, Desire Demon tries Cursed Dance (must have been resisted), Winter's Grasp... and then I am free. Health Pot, Fireball, retreat to door. Several skeletons nearby. Mind Blast. Lesser Lyrium. Desire Demon closing, so back up slightly, Cone of Cold again. Then I am tired of this so Shock bomb, Fire Bomb, Acid Flask, Soulrot bomb. Desire Demon and several skeleton's dead. Hit by Templar Shield Bash... Health Pot, lesser Lyrium then Arcane Bolt to kill Templar and then killed the last skeleton.
It was nice to beat the encounter, but this would hardly be a consistently repeatable tactic unless you include reloading to get past rng as a tactic.