The main thing here is to remain mobile, and this being an open-world boss, you can use yourSpectral Steed Whistleduring this encounter. Make sure to be on your mount as the dragon lands, and go in for a few hits. The boss will try to counter that by either stomping, doing a spin into a bite if you’re behind, or flying backwards and spewing flames underneath. While the first two are easily avoidable while you’re on your mount, the fire attack is more dangerous. Whenever the dragon leaps backwards, run in the opposite direction to avoid the coming flames.
As for a safe way to get close to him and get a few safe hits in, stay in mid-range in front of the boss. Most of the time, this will prompt the boss to do either a frontal fire attack, or one going from his left to his right. When you see him gathering flames in his mouth, quickly run in through the left side and start attacking the legs. After the dragon tries to counter-attack, run out, bait his fire attack again, and repeat until the boss is dead. Do note however that sometimes, the boss will roar and charge forward instead. You can avoid it by running backwards, but it will give you a much smaller window to attack the boss.