Three Powerful Guardians At Low Level - Fast Leveling, One-Shot Everything (Horde) (Patreon)

This exploit is horde-only and requires you to complete the undead quest chain up to "Transdimensional Warfare Chapter III"  https://www.wowhead.com/quest=27518/transdimensional-warfare-chapter-iii 

Note that this is quite a way down the undead quest chain and immediately follows another quest which is potentially very useful for exploiters. 

This exploit is useful from level 20 - 60.

When you turn in transdimensional warfare chapter 3 you are given 3 combat guardians, Lord Walden, Lord Godfrey and Baron Ashbury. These are powerful npc's capable of killing anything at appropriate level in the whole of the continent.

Normally these npc's vanish when you leave the relevant quest zone, however you can take them out of Silverpine Forest using a simple glitch: just backpedal in and out of the quest zone, on the border of Silverpine. You have to time it so that the npc's spawn as you are outside Silverpine, it takes a little luck, you will need multiple tries before you get it right and some practice-it becomes easy after you succeed a few times. 

You can just use these npc's to quest. I would highly recommend you use the npc's in conjunction with this public exploit I contributed to youtube years ago which uses the same glitch to acquire two other npc's:

 https://www.youtube.com/watch?v=QT2uNNW-6F0 

Collectively you have 5 overpowered NPC's. In BFA this is much more powerful than it ever used to be because questing is the fastest way to gain xp now, and the health of mobs is no longer as trivial as it used to be. You only have to aggro a target and the mobs will kill it: effectively you can one-shot everything.

The major limitation on these npc's is that they despawn if you hearth or port, die, use a flightpath, or phase. 

An alternative approach is to use the NPC's to farm instant respawn npc's such as the dragonkin/orcs in the east of the Wetlands.