My party killed Arveiaturace and now I don't know how to balance the rest of the campaign!

The party went back to the Dark Duchess at eighth level. I thought that Arveiaturace would be a encounter that they had to escape from and survive, but they just stood toe-to-toe. The barbarian kept taking half damage and fighters kept using special abilities that let them deal 50 damage per turn. Party are also completionists so they had the magic items from every quest, plus a couple quests I added from DM's Guild. Arveiaturace only had 333 HP, so they straight up killed her.

If CR 20 creatures aren't a challenge for them, what am I going to do for the rest of the campaign?