the first time i killed him was at coronation but ended up reloading because didn’t want to take out the grand duke (at time I didn’t know there was going to still be a way to bring him back). I took both rune powder bombs, the one I stole in act2 and the other the gnomes gave me in act 3, loaded up a buncha smoke powder barrels, had astarion drink a potion of cloud giant strength, greater invis’d him and dropped all that shit around gortash. Then I used mage hand to put a few more because greater invis was breaking after the 7th barrel drop or So. So after placing around 20+ explosives, I got up the ladder, fired a smoke powder arrow at it and one tapped him.
Did you also kill all the watchers and everything else? I did kinda the same thing where I reloaded when I saw how many maybe-innocent people were around, but I didn't follow through to see what would happen to the quest lines.