On this page of the guide to The Witcher 3: Wild Hunt, you will find a walkthrough for a side quest - Contract: Missing Miners. This is a DLC quest and the following walkthrough explains, among other things, where to start it on Skellige, where is the rock troll's cave, and how to deal with the monster.
You will start the quest by reading the notice board in Blandare on Skellige islands (M13,17). Go to Giarra after reading the note, you will find him in the building on the top of the village. Start with negotiating - he will agree to pay you 300 crowns.
Go to the miners' camp not far north from the village. There you will find some footprints that will lead you to the cave of the rock troll. Use the ogre oil before the battle. He will attack you, but after you take around 1/6 of his health, the negotiations will start. You can let him free, you will get 50 crowns and two silver ores from him and another 50 crowns from the questgiver.
After killing the troll (the battle shouldn't be hard if you use Quen sign) you can collect troll skin, troll mutagen, "Caroline" sword and cave troll trophy (5% additional experience for killing monsters). In the eastern part of the cave you will additionally find 3 types of crossbow bolts. Go back to Giarro for your reward. You will receive more experience for killing the troll than for letting him go.
Main reward for completing the witcher contract: Cave Troll Trophy - 5% more experience points for eliminating monsters.