Thus far, he's slain plenty of demons. If Tanjiro need to face all-new threats, and he'd need the full power of water breathing and even Hinokami Kagura to win. There are plenty of villains from other anime that Tanjiro could defeat if he didn't hold anything back.
The deadly martial artist named Shin is nearly a match for the classic shonen hero Kenshiro, and he may also have partially inspired Dio Brando's design in JoJo's. At any rate, Shin is a formidable opponent even for Tanjiro, but he's more used to fighting his fellow martial artists rather than swordsmen.
Tanjiro's best techniques should keep him safe from Shin's wrath, and the opening thread is the key. If Tanjiro can combine that with Hinokami Kagura, then he stands a real chance of finally taking Shin's head right off to save the day.