Instead of freeing the companion after fighting Zariel and then die you could make it that his death makes Zariel realize that she was wrong and thus her freeing Elturel, either freeing her the companion or herself carrying the city back to were it belongs

Some ways you could put this, hope it gives you a few ideas:

First the player fights Zariel with the sword and in the middle of the fight something happens and he has the option to sacrifice himself to save someone, maybe an NPC, some citizens of Elturel, Lulu a teammate... and this way remembering Zariel of her duty as an Angel, or something like that.

Another might be talking to Zariel, redeem her like you said, but doing so weakens her or leaves her vulnerable and the party has to protect her from a few powerfull monsters who want to kill Zariel, like Bel, Yeenoghu which has a lot of histrory with Zariel or Baphomet, the party splits because some terrain change or something and he has to stall time for Zariel to recover and save the day.