So you won the second time because you were lucky, as blocking the doors is one way to win.
The problem is not that the fight is hard. The problem is that it is not a normal, but a scripted fight.
The hardest mode is called "Tactician", not "Insanely Hard" or ""Crazy". You should use tactics. I don't see a way to decide the Isobel fight with tactics, except with hindsight. I have no problem with reloads if I made a mistake or there were overwhelming odds. So far I had to replay one fight, that against the captain of the Gith creche (not the inquisitor) because all enemies went first, Shadowheart was dead after that (not only downed), main char downed, Gale without most spell slots; I reloaded instead of trying to win, perhaps a mistake. So "Tactitian" is not in itself so difficult as I'm surely not the best player and mastered almost all fights. But the Isobel fight is different.
It is also different from a normal lost fight because your party did not lose. They all are ok and ready to fight, but the fight is over because of "script". It is comparable to a lost dialog check. Normally I don't reload after negative events. But here the negativity of the consequences is not bearable, for me at least.
I hope there are not many comparable situations in the further game.
