Ninetails can actually get one of its strongest moves at just level 10 with this simple method in Pokemon Sword and Shield.
While Sword and Shield launched with a wide assortment of brand-new Pokemon, players will still be forced to collect the old ones if they want to completely fill out their Pokedex.
This means you'll have to get your hands on a Vulpix and evolve it into a Ninetails, which will knock out two Pokemon off the Pokedex. While you're in the process of doing this, it might be worthwhile learning how to teach Ninetails Flamethrower so it can be fearsome right off the bat.
Keep in mind that Vulpix is a Sword exclusive Pokemon, so you'll need to make a trade for one if you own Shield instead.
Once you have your hands on a Vulpix, you'll have to use a Fire Stone to evolve it into Ninetails.
A wild Vulpix can be caught along Route 3 in Sword so you can grab one very early in the game. Once you evolve it into Ninetails, you can actually teach it Flamethrower.
How to teach Ninetails Flamethrower
Players will be able to find a Move Tutor at any Pokemon Center that will help a Pokemon learn a move if it for some reason missed learning.
In this case, Vulpix didn't have the chance to learn the move since it evolved, so this tutor will help your Ninetails "remember" the move Flamethrower.
Learning Flamethrower would instantly turn your Ninetails into a formidable opponent, so learning one of the most powerful fire-type moves at a low level is a good idea.
It's an easy way to get a powerful move early on, you'll just need to make sure you have a Fire Stone handy for the evolution process.
In fact, you can repeat this simple process with several other Pokemon that evolve through the use of a stone, so it might be worth using the same method on all of them.