All new faded for her bug

In order for the mission to trigger I used a save from before I encountered the bug. I went to the area and stood in between the two pillars closest to the coast, a few inches away from the left pillar. It only triggered for me when I was in this exact location and only after Solas' related dialogue.

As I understand it, once you encounter the bug and the mission marker disappears, the mission can't be done. Perhaps this only applies if you leave the area then return. I attempted what worked with my more recent save and the mission did not trigger, which means I've lost about 3 hours' worth of quests, which I'm not happy about, but Solas is worth it.

It's one of those quests that is sensitive to how you approach it for it to work correctly.  After you receive the quest and head to the Exalted Plains, start from that first camp and head West to slightly NW until you come to the River.  I usually aim for a Rift that needs closed (NOT the one North of the bridge, there is another Rift about mid point on the River).

From that point head South along the River.  You will come to a small ruin where you will have a small battle w/ bandits.  The ruin itself is an important part of another quest, but only as a progress marker for your current quest.  After that battle Solas will say something to the effect of:  "We're getting close now, thank you Inquisitor for helping me with this".

Proceed South until you see a road/path on your left.  This, unfortunately is very important, and the reason you should not approach this quest from the Elven camp, because you would likely miss it.  Anyway, there are some bodies in the road that you need to inspect.  You will inspect 2 of them.  One is killed by arrows, but another has claw marks.  Solas becomes upset at this discovery because it is from the Demon form of his Spirit-friend.  A short conversation ensues.

Head SW to some pillars that you can see and you will encounter the mages that summoned the Demon.  A 3 way conversation ensues between the Inquisitor/Solas/and the mages.  Answer wisely if Solas is your friend.

Go to the pillars where the Demon is and destroy the 5 pillars.  Do Not attack the Demon.  When he attacks someone in your party have them move around instead of attacking him.  Concentrate on destroying all 5 of those pillars.  Once the pillars are destroyed, a short cut scene will happen with Solas and the Spirit (no longer a Demon).  After that a short conversation and Solas will leave the party, rejoining at Skyhold.

You can earn some major points with Solas on this quest if you choose the right options.

I liked this quest, it's just unfortunate that those corpses are so closely tied to being able to complete it. 

I understand that this quest is apparently very buggy (the entire game is pretty buggy but that's a different story) and I've been looking through some forums trying to find a solution. I couldn't find one that actually works so I decided to go ahead and make a new thread about it.

I entered the area from the southwest (Dalish Camp) and went northeast, crossing the river. I turned directly north, and claimed a logging stand. Continuing north, I passed between the rock pillars on the left and the rock wall on the right. I turned to the west and continued towards the quest, with the dilapidated building and small camp on the right and the rock pillars on my left. I continued west southwest and Solas had said something to the effect of "Thank you for doing this, my friend was summoned up ahead." I continued (NOTE: I did not examine the corpses until after I left the area and came back Post-Bug) and began the dialog with the mage that had summoned the spirit. I sided with Solas and the dialog ended. We continued to the quest and I began to attack the summoning pillars. I am a rouge archer and one of my arrows had accidentally hit the demon. I figured "oh well, I'll still try to destroy the pillars" and the demon started to attack my party. Once I destroyed all the pillars the screen faded to black for a solid three or more minutes with the loading icon at the bottom right hand side. Once the loading was done, I was left at the position I was in when the loading had begun, the pillars were still destroyed, and the demon wasn't there anymore, however I still had the quest icon and the quest journal entry "Deal with the demon".

My latest save was about three hours ago and the autosave just brought me back to that same position post bug. I tried to leave the area and come back but as I approached the area, the loading screen came back and the demon was their. I tried attacking the demon but it seems like it's invincible. I left the area, saved and loaded, then cam back and nothing had changed.

Platform: PS4 Game: Dragon Age Inquisition: Game of the Year Edition

What did the Spirit say to Solas?

What happens if you stop Solas killing the mages?