Leveling Trick (20-60) - One-Shot Mobs With Vehicle (Horde, Requires Partner) (Patreon)

Some way down the Horde Stonetalon Mountains chain is the quest "To Battlescar". This gives you a demolisher with which you are meant to kill a number of the many alliance NPC's in that area. The demolisher itself does not award you any experience, however it has a very interesting property that you'll notice if you group up with a low-level player. 

Normally any kind of damage or even aggro caused by a high-level player reduces the xp gained by any low-level player almost to zero. For some reason the demolisher's contribution does not count-this means the low-level player just has to tag a mob and the demolisher can then one-shot the mob (it does a lot of damage). Unusually for a highly optimized leveling strategy this is actually quite a lot of fun, at least for short periods. 

This is best done in a group with a friend, but obviously multi-boxing works well also. 

The night elf mobs in the area are not instant re-spawns but there are so many of them you don't run out of hostile npc's easily.