I managed to finish the fight on my 1st attempt with a level 4 party that finished everything above the ground and the temple of Selune (no Underdark) and I didn't feel like the fight was that difficult.
Party was:
Warlock Tav
Laezel (battlemaster)
Shadowheart
Astarion (thief)
I started the fight with my entire party on the ground (so no sneak attack cheesing).
I think I had 1 character dying in the fight which was fixed by an healing word.
It was one of the few fights were I used different consumables (haste potions, scrolls, poison,...).
I assume that your party is level 5 when attempting the fight after launch, so I don't feel like the difficulty level is wrong.
