There are 10 demon slayer ranks, Tanjiro being the lowest was able to put some fight against Lower Moon Rui. Which demon slayer rank is needed to be able to defeat a Twelve Demon Moon Lower Moon?
I think it's not depend on demon slayer's rank like Tanjiro. He has abilities and skills adopt from his father and his experience and succeed from Sakonji Urokodaki's training. Higher rank than Tanjiro has only have more experience in battling with demons than him that's why they're high rank. Therefore, it depends on demon slayer's skills and abilities.
Ranks are given based on completing missions so they might be a bit unreliable.
A better answer could be that, usually, only Pillars can take down demon moons unless a talanted rookie comes in picture
There are such those who reached pillar levels in 2 months after picking the blade, Gyomei and Muichiro are two such people.
In the Manga Vol 6 Chapter 47 Ubuyashiki mentions that the pillars are the strongest and most talented demon slayers in the corps, they may even defeat the 12 demon moons. In the anime episode 23, Ubuyashiki tells Tanjiro that the pillars have themselves defeated pillars to come where they are today.
Muzan mentions during the lower moon meeting that Upper moon's haven't dies for hundreds of years whereas lower moons, again and again, fail to kill Pillars and instead die by their hands. Something similar has been said about the pillars, all other ranks usually die through the hands of demons and pillars alone hold the organisation together.
Spoilers about Tanjiro and party's ranks:
According to Murata, Tanjiro and his buddies have climbed the ranks like monsters (Chapter 134). As of the latest chapter (chap 173) all 3 of them are at Pillar strength and can effectively fight against upper moons (Zenitsu soloing Upper moon 6, Tanjiro and Giyuu taking on Upper 3 and Inosuke and Kanao holding their own against upper 2)