Mob In Elwynn Forest That Makes Vengeance Demon Hunters Invincible (Patreon)

The NPC "Guard Roberts" (48.0, 67.6) respawns instantly and can be consistently killed by Vengeance Demon Hunters making them effectively invincible. Other classes may be able to achieve the same effect if they can get their leech high enough. 

This is also possible to some extent with target dummies in Stormwind/Ogrimmar. Warlocks especially can go into affliction spec and dot mutliple target dummies and have infinitely regenerating shields making the winning of duels easy.  Guard Roberts has the advantage of proximity to Goldshire without the disadvantage of level 110 guards constantly spawning.