Takin' Down the Beast

The more complicated instructions no longer apply on live. There is no quest to pick up, no tools to collect, and the ballista does not need fixing; you just need to take the bolt to Wingsmash.

Find a Dredhollow Bolt (laying in random locations around 43,77). Take it to Wingsmash (46.3,77.8) below the area with the bolts, turn in the quest (blue question mark over his head) and he will walk slowly over to the ballista and fire at Harika the Horrid. Shortly after, the rare will land. Wingsmash tanked the rare for me, but there was an interruptible spell (Vampiric Consumption) and both a pull in and knock back effect that I mostly ignored (definitely make sure your back isn't toward the edge). Wingsmash can be knocked back but it does not appear he can actually be knocked off the edge. I never let Vampiric Consumption go off, but according to its page here on wowhead, it drains the health of nearby enemies to heal the rare.

I was not fortunate enough to get the mount, but I did get the following loot:

Infused Ruby x18
Pallid Sinuous Hide
Concealed Sinvyr Flask
Grateful Offering x3
Shrieker's Voicebox

Takin' Down the Beast is a quest given by [npc=165325] to kill [npc=165290].
[npc=165290] is located in the Revendreth The Banewood zone.
Rare is flying near the cliff South of Wanecrypt Hill flight point.
It is immune to damage until you shoot it with [npc=165324].
First you have to repair the Ballista, bring 2 [item=176396] to [npc=165325].
You can find the tools inside the north building of Dredhollow
and in the cart outside of the building.
To use [npc=165324] you have to bring [item=176397] to [npc=165327].
[item=176397] is inside the castle west of [npc=165324] near the wall.
Has a chance to drop [item=180461].

Rare is a part of Adventurer of Revendreth achievement.

/way 45.83 79.34 Harika the Horrid
/way 43.23 77.62 Dredhaven Bolt
/way 40.77 72.70 Dredhollow Tools
/way 41.18 74.67 Dredhollow Tools
/way 46.30 77.86 Wingsmash

Harika the Horrid video: https://youtu.be/zxA7zfXg8P8
Dredhollow Tools: https://youtu.be/AyIEodWam80

For some reason I was unable to take this quest with warmode on. I switched to warmode off and I was able to complete it.