Virtually Invincible in Orgrimmar/Stormwind, Tank Faction Bosses/All Faction Capitals (Warlock) (Patreon)

 The following exploit allows warlocks to be virtually invincible in capital cities. At the start of Legion, both me and the youtuber Warlockolol independently published a simple version of this method but both of us missed a crucial detail which limited its power in practice. 

The exploit depends on this talent:

Absolute Corruption Talent Requires Warlock (Affliction)
Requires level 30 Corruption is now permanent and deals 25% increased damage. Duration reduced to 24 sec against players. 

(note this exploit does not require you to be in affliction spec more than temporarily). 

The warlock takes this talent then goes to an area with many target dummies, especially low-level target dummies. The warlock dot's each of the dummies with corruption. These will last indefinitely. Each dot will generate a shield for the warlock with soul leech up to 15% of normal stamina. This refreshes almost instantly making the warlock very difficult indeed to kill.

That was where I'd got up to you until recently. You can solo most of the faction leaders using the technique provided there are not too many players around, and usually they are not.

With Stormwind and Orgimmar however there's a problem: provided you receive less than 15% of your health damage in a single tick your health won't go down-so for example you can take any number of guards who do low dps individually. However, in the faction capitals you will receive much more damage than this as multiple players attack you. Saurfang also has a special whirlwind attack that does almost a million damage per second and wiped me out fairly quickly.

This was the point at which I stopped researching this exploit as it seemed it could not be taken further.

Developing the method: leech gear

However very recently I had heard on bug report there was a warlock attempting to solo Saurfang-I don't think he was successful but I believe he was tanking many players and Saurfang simultaneously for long periods. I know this wasn't possible using the above method so I began looking into the matter again.

I tried experimenting with leech. Leech is a secondary stat which restores some % of player health based on the damage they cause. Initially I'd been put off because the only source of leech for a warlock is specific pieces of armour on the ah. These pieces can be quite expensive. Moreover the amount of leech on a given item when it exists is usually tiny. 

However, I had a revelation: with all the corruption you can put on each dummy, especially with the huge damage bonus multiplier on low-level dummies even a tiny % of leech would translate into a huge amount of healing in addition to the soul leech shield.

And so it proved. There are multiple sources of dummies in Orgrimmar. In particular there is a huge concentration on multiple floors at 74.4, 47.6 and more in the nearby hunter area. Doting all of them even on my mediocre-geared lock (ilvl 914) with just one leech piece (+360, 2%) I was getting healed between 100-600K in addition to the shield. 

I was able to tank multiple players, guards, other npc's and even Saurfang when using my Voidwalker. When I attempted to dps Saurfang the Voidwalker lost aggro - which was fine until Saurfang unleashed his whirlwind, which exceeded my leech heals + shield. It is apparent however that a player with more leech items and/or a higher ilvl could almost certainly survive indefinitely. Killing Saurfang would take a long time but is definitely possible especially later at night. 

I'm not sure whether this would be viable to try and kill Anduin in Stormwind due to the lesser number of target dummies.