Adventurer, to annihilate the Dark Lord, we must make a treasure called a Demon Shirt. It's not something you can get at the Gap.
You must collect the materials required to make it from the Horror Demons and Soul-Devouring Demons in the West Branch of the Polar Palace.
You need Demon Skin and Demon Horn. A skilled adventurer like you shouldn't have a problem with this quest.
When you put on the Demon Shirt, you will appear as a demon. When you fight with Vani, it will seem like a fight between demons, so the Claws and Fangs will not attack you.
If you have all the materials you need, you can start making it now.