Potentially Limitless Damage (Monk), Advanced Method (Patreon)

This method is classed as advanced, meaning that only experienced exploiters should attempt it. Warning: This type of exploit can be very frustrating to execute. 

The Brewmaster monk 4-set tier bonus* "Keg Of The Heavens" deals an additional 50% damage, it also heals you for 66% of damage dealt, and grants 66% of damage dealt as maximum health for 10 sec. 

Bonuses to damage seem to allow this buff to health to scale infinitely. I tested this with the endlessly scaling damage debuff, from the Essence of Anger, in the Reliquary Of Souls encounter in Black Temple. (This is one of a number of high damage buffs or debuffs in wow instances). Sure enough by allowing the debuff in this encounter to scale up for ten minutes and executing Keg Smash I was able to  increase my health from 66K to 238K. You can go a lot higher, obviously, that was just an experiment to establish the proof of the concept.

The reason health buffs are interesting is that many damage abilities are based on player health. In theory with infinite health any damage ability based on health should one-shot any raid boss in the game.

The monk abilites which are based on player health are Touch Of Death 

https://www.wowhead.com/spell=115080/touch-of-death

and Invoke Niuzao.

https://www.wowhead.com/spell=132578/invoke-niuzao-the-black-ox

Keg Of The Heavens stacks to 10 and last ten seconds. It is possible to build up a massive health pool by keg smashing in an older instance with a huge damage buff as part of some old boss mechanic. Then use a lock port to a raid boss, now one-shot the boss with Touch Of Death. If the player's health pool exceeds that of a raid boss Touch Of Death should be useable.

In practice this approach has a number of practical issues: for example actually going through a loading screen with enough of the buff left on the timer to use.

Here is an alternate method which is more practical but also more complex:

1. Change Brewmaster Monk to Necrolord covenant (this is an instant process with no penalties). 

2. Go to instance with huge damage buff (eg the aforementioned Black Temple encounter).

3. Pull trash mobs to source of damage buff.

4. Use Keg Smash allowing Keg Of The Heavens to create a massive health pool for your character.

5. Use the Necrolord Fleshcraft shield ability. This will temporarily (the shield lasts two minutes) make your character virtually invincible. 

6. Now go somewhere where doing a lot of damage is useful eg raid boss. 

7. Now find some way to receive a huge amount of damage. This could be as easy pulling all the trash and a boss at the same time. 

8. You will receive a huge amount of stagger damage. Use Invoke Niuzao.

Purify the stagger damage. 

9. Niuzao's stomp ability will now scale to the amount of stagger you purified. I have not as yet found an upper limit on the stomp damage. 

The advantage of this method is that it completely bypasses any loading screen issues. 

*Note that the 4-set is MUCH easier to obtain than in previous patches. By using the creation catalyst in Zereth Mortis you can convert items purchased in-world from Sandworn relics into items with tier bonuses. This also works with LFR gear, and pvp conquest gear. I managed to obtain a 4-set from my monk in less than two days play.