This Shadow Talent Build is the fastest for Leveling Up a Priest Class from 1-60 in WoW Classic patch 1.12. Spamming Shadow Damage DOTs while kite mobs will be the name of the game with this build, having the ability to sustain your health is just a bonus. You will just need to make sure you are stacking enough spirit so you don’t run out of mana often as that will slow down the levelling process drastically.
It seems the first stress test didn't quite do the job yet, as Blizzard are scheduling a second one next week, due to more issues with the first one than were anticipated (mainly of the login screen variety). Everyone that was invited to the first one will also be invited to the second, the level cap will be raised to 10 and it will also be available for a full day, starting at 2pm PDT on Wednesday, May 29th.
2. Achieving. There are many pet battling achievements in game, and following the natural trail through the Taming [Continent] achievements up to Pandaria, and then on to the Spirit Tamers, Beasts of Fable and the Celestial Tournament was an epic journey for me and for many others. It's well worth doing. The Celestial Tournament is still, IMO, the pinnacle of PvE pet battling. However, WoD and Legion and BfA have added more battling achievements.