Hurdle

Go to the entrance of Stormterror's Lair: You'll find the lair to the west/northwest of Mondstadt. The quickest way to get there is to teleport to the Cecilia Garden Domain. From there, head west to hop on the path, and then follow that path north. You might run into a few enemies on the way, but honestly, you can just sprint past them if you're in a hurry. As you get close to the barrier, a cutscene will trigger. Venti will start talking about breaking through, but he gets interrupted by an enemy attack.

Defeat the assailants: You're up against a Wooden Shield Mitachurl, an Anemo Samachurl, a Hilichurl Grenadier, and a couple of Shooters. Your best bet is to take out the Anemo Samachurl first – its attacks can really mess with your ability to fight the others. Once it's down, deal with the rest of the smaller enemies, and then focus on the Mitachurl. Make sure to use Pyro attacks on its big shield; it'll break eventually, making it much easier to finish off. After you've cleared them out, another scene will play. Venti will do his thing with the harp, and poof! The barrier disappears. With the path clear, this quest wraps up.