I don't understand why people have complained about this fight and I never really have done. My first run was on Nightmare and I had zero problems.
What I do (I race to unlock specs, then return to old content) is race through prologue, race through Hinterlands to get 4 power, go to Val Royeax and buy all the tier 2 schematics. Twink out my party, do either the mage/templar quest line then do In Your Heart You'll Burn.
My last set up was probably the easiest attempt yet. I had both Cassandra and Iron Bull as s/s tanks, myself as a rogue archer and Solas as a support mage. It was quiet simply a taunt fest (both set up with challenge and war cry preffered with shield wall preffered, everything as disabled, manual control of Charging Bull), and in combination with Solas's barriers, neither tank was remotely in danger. My rogue archer just followed the cycle of Hook & Tackle -> Leaping Shop -> Wounding shot (the very last shot in the archery tree), rinse and repeat. Takes about 3 waves until the Trebuchet is fully powered.
Think my hardest one was probably my first play where I played a mage, didn't spec my tank right nor my mages. My main advice point is if you build your tank right (which really isn't hard, a simple tank build that is always effective is war cry/challenge/s&s passives/shield wall. With that alone he will not die, all you have to do is manually target Charging Bull), you won't have trouble at any point.