This quest is currently near uncompletable. The dragons you fly on run out of mana after 2 shots and the ballistas take about 10 shots to kill. You die almost instantly if the ballistas hit you. There are several threads on reddit about this quest and people are posting about it on the DK forums.
Please do something to resolve this.
There’s also bugged quest in remix where u need to kill monkeys as a NE. Same trouble u need a plenty hits to kill em but u die in 5-6 hits. Blizz as always 
Hey Unknòwn,
I can confirm that our developers are aware of this issue, and are working on a fix. If you encounter another bug in game, please do report it to our developers, and they’ll be able to check this out for you. 
Bug fixes are usually published in our Hotfix blogs and patch updates, which can be found in the News section of the main site.
Thank you!