Upon killing Professor Slate in the revamped version of Scholomance, you can loot an Empty Polyformic Acid Vial. This can be filled from a vial on Slate's desk for a three-dose vial of the Acid, which transforms you similar to other items such as Savory Deviate Delight and the like.
Once obtained, simply head to the dungeons with the aforementioned bosses, dose up beforehand, and take them out! The required bosses can be found in the following locations;
- Commander Ri'mok is the third boss of the Gate of the Setting Sun instance,
- Liu Flameheart is the third boss of the Temple of the Jade Serpent instance,
- Gu Cloudstrike is the first boss of the Shado-Pan Monastery instance,
- Trial of the King is the first encounter in Mogu'Shan Palace,
- Vizier Jin'bak is the first boss of the Siege of Niuzao Temple,
- Yan-Zhu the Uncasked is the third boss of the Stormstout Brewery
Note that only one Empty Vial will drop per kill of Professor Slate, so it's probably easiest to solo the normal (level 40-ish) version of Scholomance to get your vial. You'll need to do so twice, as each vial is good for three doses.