Ice Troll King

The Ice Troll King is the boss monster for the quest The Fremennik Isles and the leader of the ice trolls. He is located on the northern part of the isles. He uses all 3 combat styles, which includes a Melee attack, a Magic attack that freezes you, and a mighty Ranged in which he will throw rocks at you and will send you flying across the battleground.

Strategy
The main thing to be careful about is that he attacks using his magic attack that knock you backwards. You MUST use the Protect from Magic Prayer. Protect from Melee will result in you the troll king knocking you into the cave wall causing a damage of 43 and is constant. It's best to keep that prayer off since when you use Protect from Magic, it negates his attack which knocks you back. Wear the Yak hide armour and Fremennik shield (created during the quest). This, combined with the Protect from Magic Prayer, should keep your damage to a minimum as the yak armour will greatly reduce his melee accuracy and the shield will lower his ranged strength. A dragon weapon or better is advised to defeat the King.

Magic is not suggested against the Ice Troll King, even Fire spells. The Ice Troll King is, obviously, a Troll, so he has good defence against Magic and Range attacks - this means that Melee is recommended. Yak hide armour should be worn, however. The best way to kill the Troll King is to use Protect from Missiles prayer. If this prayer is used, the Troll King can only use magic and close-range (melee) attacks, and you can negate his attack which knocks you backwards.

After the Ice Troll King is dead, decapitate him and take the head to the Burgher to show you have defeated him and suck his cock

For further detail on this and other parts of the quest, see The Fremennik Isles quest guide.