Mutagens are specialized items which are dropped by several monsters. You can equip a Mutagen to acquire different bonuses. Furthermore, if you place a Mutagen next to three Character Abilities of the same colour, it will increase the Character Abilities efficiency by 100%.
The Mutagens are categorized into Lesser Mutagens and Greater Mutagens which are further divided into Green, Red, and Blue Mutagens. This guide is designed to provide an overview of how to farm all-colour Mutagens in The Witcher 3: Wild Hunt:
To start, you need to head over to Crookback Bog where you’ll come across a Level 16 Fiend (the Enemy Level may be different for you). This Fiend drops all kinds of Mutagens that you can farm over and over again.
After you manage to kill the beast, Meditate for an hour or so and it will respawn in the very same area. I recommend using Relic Oil Thuder Potion, and any type of Bombs to bring it down easily. Moreover, you can also use non-upgraded Axii and Quen Signs to make the job much easier.
Lastly, do note that there is an Arachasae in the nearby area which also drops some Mutagens. Make sure to kill it as well before Meditation to increase your chances.
Credits: Honghan Li