As is prone to happen in The Witcher 3: Wild Hunt, Geralt finds himself amidst politics and in the position to aid monarchs. On Skellige, it is possible to pick up a secondary quest from Kaer Trolde from Crach an Craite, the Jarl of Skellige, which involves the player aiding his son, Hjalmar, in defeating an ice giant for glory and renown.
Hjalmar an Craite seeks to prove himself as the right choice for the next Jarl of Skellige, which is the entire reason for his quest, though he may be out of his league. Enough that his father asks Geralt for help in making sure that Hjalmar returns alive and whole. It is in the player's interest to accept, despite the major effort, as there is quite a lot of XP to gain, as well as other rewards, from this giant hunt.
Step #1: Arriving On The Island Of Undvik
When ready, hop onto a boat and sail to the easternmost island of the Skellige archipelago, for this will be the island of Undvik. If one has already been here, fast traveling is advised for convenience. After making landfall, follow the quest marker until a cutscene plays, showing an ice giant feeding flesh to some sirens as well as indicating someone is within the nearby ship. This will grant the player 100 XP.
After the giant leaves, enter the ship and speak to the man one finds inside. This person, Octo, is a bit mentally unstable and recounts the mythical, and false, goals of the ice giant. However, Octo will also have useful information, so be sure to exhaust his dialogue to gain a strong hint about where to find Hjalmar. After this, leave the addled man to his fate, as he doesn't really have anything worthwhile to give Geralt, and follow the new quest marker to the an Craite camp mentioned by Octo and use one's witcher senses to spot and follow some tracks.
Step #2: Rescue An Archer From Becoming Troll Soup
At the campsite, there will be two kinds of tracks: footprints and the marks left by a boat being dragged. Follow the footsteps first, for their will lead to some important information about Hjalmar's whereabouts as well as an interesting situation. The footprints will eventually lead Geralt through an abandoned village and into a cave, where the witcher will come across some ice trolls heating up a cauldron in which they are planning to use to boil a captive archer and make him into a soup.
When Geralt engages the ice trolls about what they are cooking, the player can straight-up just fight them or play a guessing game with them. It is recommended to do the latter because of the hilarious dialogue. If the player successfully stumps the trolls (which is not difficult at all), Geralt will be able to leave freely with the grateful archer named Folan, which will grant 200 XP. When clear of the cave, talk to Folan and he will give the player information regarding Hjalmer's company and will join Geralt's party for the remainder of the quest. Now, head back to the old camp and follow the tracks left by the boat.
Step #3: Travel Up The Mountain
Follow the boat's trail until it is found next to a dead body. Proceed from here to the nearby village of Urskar, where Geralt will then find more tracks that lead into a cave. When approaching a ledge, Geralt will state that the water at the base of the drop seems deep enough for a safe dive, which is a hint for the player to jump into the underground lake. After taking the plunge, get out of the water and continue following the tracks.
Along this trail, Geralt will find corpses of Hjalmar's companions, which bodes ill. Folan commenting on the dead bodies will give context and confirm that the pair is on the right track. The duo will soon come across a cavern with a tree guarded by harpies, which should be cleared out before proceeding up a nearby ladder. After ascending, the player will find themselves in a passage that will lead them out of the cave system to Clan Tordarroch's Forge. This abandoned structure is where the ice giant stores his food and within provides clues that will help one find Hjalmar.
Step #4: Finally Finding Hjalmar
Enter the wooden structure on the mountain and a cutscene will play showing the gruesome interior packed with flesh and corpses. Folan will state that none of the remains are Hjalmar's, though, the relief is short-lived as the pair are assaulted by undead enemies.
After felling these decrepit foes, exit the building and follow the tracks until they separate into one trail of human footprints and another clearly made by the ice giant. Follow the human-sized ones and the quest data will update, granting a new marker. Using this as a guide, head in the direction indicated until one reaches the Dorve Ruins, where the player will find Hjalmar in the middle of fighting off a bunch of sirens. Help the lordling defeat the monsters and a conversation with him and Folan will ensue.
Step #5: Slaying The Ice Giant
After speaking with Hjalmar and Folan, Geralt and his two allies can proceed to the lair of the huge monster. Upon reaching and entering it, the player will find the frigid and mighty Lord of Undvik sleeping. Nearby, there will be an NPC named Vigi the Loon trapped in a cage. Hjalmar insists on leaving him, though players should ignore this and rescue Vigi for the bonus 200 XP he provides upon his release.
To free the Loon, Geralt must sneak through the giant's lair and collect the key at the far end while avoiding stepping on snow, as the crunching sound this makes will wake the Lord of Undvik. When freed from his cage, Vigi will immediately run up to the giant and kick him, waking the monster and initiating the boss fight. This will be a tough battle but with Hjalmar, Folan, and Vigi all backing him up, Geralt has a decent chance against the ice giant; especially if he uses sound witcher tactics.
The key to defeating this behemoth is the liberal use of Quen as well as applying the strongest possible version of Ogroid Oil to one's silver blade. The giant's attacks hit very hard, though are relatively easy to dodge. However, be wary as when it loses about 33% of its health, it will begin using a ship's anchor as a massive flail. After the monster is defeated, the player will receive 100 XP for slaying the (former) Lord of Undvik as well as 500 XP for completing the quest.