Comments posted to our Dark Souls 3 Wiki
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Frankly, this is a stupid boss. I didn't read the messages on the ground, noticed that I did only 16 damage to his feet with each swing. I nearly killed it normally, using the Ithyrill sword +5, because Frost effect would trigger normally on Wyvern's neck. I realised what I had to do when it's health was around 1/3, or even less. I ran for it. I walked off a cliff accidently. FML. Killed it easly on second try, though, despite the annoying little enemies.
I was about to run out of estus while fighting this and the man-serpents, so I didn't bother to try exploring the upper levels. I never knew that a plunging attack was possible! So I took it out just like how people used to hunt dragons. I equipped my Milwood Greatbow and sniped it using Dragonslayer and Lightning greatarrows. Had to dash from cover to cover, and shot it while breathing fire in another direction. Took me about 80 greatarrows. Missed about 1/5 of the shots tho because of the head which always sways. But after about 30-45 minutes, you'll get the satisfaction of shooting down a F*cking Dragon!
Ancient Wyvern - "How do you like gimmicks, friend?"

Me - "Not so much my guy, not so much."
Keep in mind, if every boss were a straightforward combat test the game would be boring as all hell. Souls games have always been a mix of pure combat skill & pressured problem solving skill. A balance of might and wits. If you don't have the wits, you have the wikis to fall back on.
Easy boss if you got a frost weapon. The frost triggers immediately if you hit his head and does relative high dmg. His moveset is pretty basic so dodging him shouldn't be hard. I found that staying to the left of his head makes his breath attack useless and makes it easy to anticipate his other attacks. Still wasted all 15 estus flasks but at least it's more fun than oneshotting him...
Or... you could just use the move that was passed down from Dunkey to Dunkey. The plunging attack
First time fighting this boss, I was a Faith caster and found him easy to face simply by standing out of range and spamming sunlight spears. Second time was with dex build, and though the plunging attack adventure was annoying, it was still fun. Helped that I already knew where to go.

Just wish lightning great arrows did more. For a weapon designed to take down dragons, a measly 340 damage is disappointing.