The Troll mutagen doesn’t seem to drop always from trolls you encounter in the game. More so, the troll you have dialogues with or ones that are part of the quest don’t seem to respawn again. I was level 35 and still haven’t got troll mutagen and was even lacking Troll liver to upgrade my remaining potions.
Fortunately, there is a Rock Rabid Troll that does indeed respawn whenever you meditate.
The troll is located south eastern part of Velen, near the Crossroads waypoint, on the bandit camp. He is level 10.
Getting The Troll To Respawn After Meditating
To respawn the troll after meditating, kill it first then save your game. Then reload that game. After reloading, a new troll should appear. Now if you want it respawn again, all you have to do is meditate for 1 hour. The troll should respawn somewhere on the same area. You can see the video below where I saved and reload the game, then meditated to spawn the troll.
The troll can drop Monster Brain, Monster Tooth, Monster Blood, Monster Ear, Monster Heart, Monster Saliva, Monster Eye, Monster Hair, Monster Bone, Troll Hide, Cave Troll Liver, the Troll Mutagen or Infused Shard.
You might need to kill him a couple of times to get the Troll Mutagen though. But nevertheless, it becomes easy with the above method.
Skellige
In Skellige Isles, there is also a level 16 troll that respawns, but only if you have completed certain quests beforehand. The troll is located at the Trail to Yngvar’s Fang waypoint. It is on the north eastern corner of the Skellige Isle map.
You need to have done 2 quests to spawn the troll though:
1) Tower out of Nowheres (lv30) – located on the tower west of that same island. This seems to trigger The Path of the Warriors quest.
2) The Path of the Warriors (lv.16) – this is the quest that will spawn the troll on the said waypoint after completion.
Just like above, to spawn the troll, kill him first, then save/load the game. Then you can just meditate one hour afterwards to keep him respawning.