The one in the base game and not any of the DLC?
Well, assuming you talked to it first before attacking it is an optional battle and gives a pathetic reward. If you are determined to beat it, rush straight at it at the start and wedge yourself between the toes of its back feet, between the big toe and the middle toe and hack away. If it lifts up its foot to smash you, roll over to the other back foot and wedge yourself between the equivalent two toes and hack away at it. Keep alternating between back feet as it tries to crush you with it.
Stick to 2 or 3 hit combos as you will want to maintain enough stamina to run. If he rears up to take off, immediately run toward the shadow of his tail to avoid the fire. If you run in pretty much any other direction you won't be able to outrun it. Sometimes he rotates horizontally when flying up so turn left or right to keep running toward the tail. Make sure you are not directly under the shadow when he drops down so the tail doesn't crush you. If it sticks its head under its body and fires a blast of fire backwards, feel free to attack enough times to deplete your stamina gauge once as the animation is long and the fire can't hit you between the toes.