The fastest way would be to buy a character boost to 90, but you're probably looking for a different alternative. I have found that doing Dungeonbuddy with RAFd accounts is very efficient, with one of them as healer. Otherwise I would personally recommend using kicks levelling profiles, which are getting better and better. With heirlooms and good flight speed, you can also level with ArcheologyBuddy, this is the most AFKable solution and quite profitable, although also slower.