Easy Greater Mutagens
The drop rate for Greater Mutagens is really low. But you can get any Greater Mutagen you need within 10 minutes. Dropping two points in the Side Effect skill will increase your drops by 10% when you create potions. At first this appears to just net you Lesser Mutagens. To get Greater Mutagens, buy a few formulas for potions that only require a couple of ingrediants. The Grapeshot is easy to make, it only requires two. Gather a ton of ingrediants and save your game.
Meditate and select the potion/bomb you want to make. When you create the potion only make one at a time. The 10% side effect of creating a Mutagen applies to every time you create a potion or bomb. By creating one at a time you are increasing your chances of getting drops. Within 10-15 tries you should see Mutagens pop up. There will always be multiples created. Usually 4 or more. Sometimes as many as 10. Most of the time they will be lesser or normal. If you create 20-30 potions/bombs and still haven't seen Greater, reload your save. Check your inventory often. Sometimes the text is difficult to make out and they can go unnoticed. When you create Greater Mutagens they always come in groups. You'll get 3-6 at one time. Once you get a handful, save your game. Rinse. Repeat. I did this 5-6 different times and each time was able to get 5+ Greater Mutagens within 10-15 minutes. I had 20-30 extra at the end of the game on my Dark run that I didn't need.