I captured this during my SL1 run. I got his tail weapon (the Obsidian Greatsword) a few tries earlier but was killed shortly after. I always go for his tail first because it can do a lot of damage later in the fight if I don't (there are two spots in this video where I could have been hit by his tail which would have proven fatal to my SL1 with a red tearstone ring). The trick is to provoke his swooping attack. Roll once away to avoid the attack, then once or twice back to position yourself right behind him. Unlock your target lock. He will turn his head/neck to find you and attempt to slam his tail down on you. I always take a step or two away from the tail at this point to make sure I don't get hit. When he slams his tail down, hit him with a chopping weapon of your choice or, as I did, Great Combustion. I only needed one shot with the RTR, the Bellowing Dragoncrest Ring, Dusk's Crown, and a fully upgraded pyro glove. It may have taken me two shots when he dropped his weapon, I forget. Once his tail is off, the fight gets a lot easier. Hope this helps.
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