if your character dies, there's no saving that character AND, this character has to be Lae'zel, because you want to play the game based on that storyline, what would you do?
Well ...
Depends on how much you would be willing to cheese it.
First step would be level 5 ... i mean, this is nobrainer.
As i said Metalic Helmet is must have, since then NPCs cant critically strike you ... wich helps a lot with survivability.
Ofc, if i visited Grymforge allready, Adamantine armor helps a lot, since if you get hit ... your attacked get reduced chance to hit you again ... prefferable piece here would be Helmet:
https://baldursgate3.wiki.fextralife.com/Grymskull+Helm ... since you can then still wear Githyanki armor and have more AC.
There was quite good suggestion here to give her a Shield, to raise her AC even more ... it requires a bit of metagaming, or ignoring her bonuses tho ... since Lae'zel is more Great Weapon type. But anyway ... prefferably, this shield:
https://baldursgate3.wiki.fextralife.com/Glowing+ShieldI would also recoment boots that gives you few bonus HP, when you heal:
https://baldursgate3.wiki.fextralife.com/Boots+of+Aid+and+ComfortAnd resistance you can get from neck as suggested abowe:
https://baldursgate3.wiki.fextralife.com/Amulet+of+the+Unworthy ... in case the gloves allready work as inteded.
So ... just out of this, for Lae'zel herself, and no help from outside:
She would have if i count corectly 19AC (15 Armor, 2 Dex, 2 Shield) ... could not be striked criticaly ... if hit, attacker would get -1 to another attack ... resistance to Slashing damage ... and if even trough all this they would reduce her to 50% HP, she would automaticly get 12 Temp. HP ...
I would say that is quite good survivability.
Then you can boost it (as you surely know) with some magic preparations:
Basic stuff you should allways cast on your party aftear each rest is Aid:
https://baldursgate3.wiki.fextralife.com/AidAs you said, Shield of Fate is great spell ... and if you wish to make sure that Shadowheart (or your other cleric) will hold concentration, its nothing easier than leave them up on the bridge:
https://baldursgate3.wiki.fextralife.com/Shield+of+FaithSince you said your Lae'zel was Eldrich knight ... i would certainly recomend boost her starting HP with False Life (or use Scroll):
https://baldursgate3.wiki.fextralife.com/False+LifeYou can also summon Shovel and hope that they will attack him, its a weak defense but every bit counts.
https://baldursgate3.wiki.fextralife.com/Scroll+of+Summon+QuasitDrinking some potion of resistance is also not a bad idea ... but wich element? Your guess is as good as mine, fire damage is said most common one.
And quite honestly i dont really remember if duration of spells during dialogue is stoped (but i think it is) ... but if you wish to be extra sure, you can also cast Scroll of Mirror Image on yourself:
https://baldursgate3.wiki.fextralife.com/Scroll+of+Mirror+ImageAnd if i didnt miss anything ...
I believe thats it! This is the most Tankish Lae'zel you can make before the battle even starts ... if she dont survive like this, you were very unlucky.
Oh wait! There is one more tiny little thing.
If you play Fighter, or Paladin yourself ... and are using a Shield ... and have Protection:
https://baldursgate3.wiki.fextralife.com/ProtectionYou can use that aswell to make her survive, since your Tav have to go to that conversation aswell.
And ofc ... there is that option to simply talk out of the situation.
That is certainly safest way.