Sadly that seems to be a bug when you are too good...
She is supposed to flee and you to follow her shortly before she has no more LP.
I suppose that this happens when there is a spell on her (poison, soulmagic, ...) "killing" her after the fleeing has been triggered thus rendering her "immortal".
The only way around it is to start the fight anew from a point before she gets "immortal" (my last savegame before the fight was the autosave of the arrival, I hope you have a later savegame...)
Sorry for the bad news!
btw: you have to kill 4 titans to get there, but that is a spoiler in itself!
Thanks for the answer. I've done everything to kill Zara, i went to the beach again and talked to everyone, went to the elvish site again, even let her kill me. lol.
I think that i have a savegame after i took the key from the elves. ( At least i hope so.) Thx again. Back to the game...
I played the endgame 5 times now and had that error once (but is has been confirmed by others).
At the time the bug occured I had only the autosave and the quicksave from the last round, so I had to fight the bridge-fight and all three factions anew.
But it hardly makes (technical) sense that the bug is "active" before the Icegolem has been spawned, so you should be lucky!
I know where i failed this time. (I took 4 potions instead of 3 , i should have known that there would be consequences for this.), but any help would be more than welcome for the other tasks.
The other 2 are way more simple and obvious: saving Larissa (or her sister, I constantly mix them up...) and taking Angelinas "side" at the elvenpost against Sweety...