Restore Kangaxx's Body - Baldur's Gate 2 Side Quest
Restore Kangaxx's Body is a side quest that begins by talking with Kangaxx himself in the Docks District of Athkatla. You can find Kangaxx inside of the building adjacent to Mae'Var's Guildhall in the southern portion of the Docks District. Check out my Docks District Overview page if you need help finding the specific building in question.
For the first part of this quest Kangaxx asks you to recover parts of his body which are currently in the possession of other Lich residing in Athkatla. The Golden Torso can be obtained by defeating the Elemental Lich in Bridge District and the Golden Legs are found on the Shade Lich in the Cult Hideout map under the Temple District.
One thing to keep in mind is you won't be able to obtain the Golden Legs until you progress the Investigate and Destroy the Cult side quest far enough. You'll need to speak with Gaal and agree to help him, he'll give you a key which will unlock this part of the Cult Hideout map.
Golden Torso: You can find this piece of Kangaxx in the Bridge District of Athkatla. There is a building south of Delosar's Inn which houses the Lich that drops this item along with another door that leads to the Twisted Rune Hideout. If you need the exact location check my map on the Bridge District Overview page.
Golden Arm & Leg: This part of Kangaxx is found in the Cult Hideout map of the Temple District Sewers. In order to reach the part of the sewers with this lich you'll need to progress the side quest Investigate and Destroy the Cult far enough.
Elemental Lich in Bridge District
Shade Lich in Temple District Sewers
If you struggle with either of these 2 Lich enemies I recommend you check the wiki walkthrough for Shade Lich and Elemental Lich. As you probably already know, most Lich enemies in this game are extremely powerful and can use very high level magic. In order to beat them you'll need to have powerful magical weapons and be able to use high level spells yourself. All Lich are immune to spells Level 5 and lower.
After defeating each of these Lich enemies to get the body part of Kangaxx you'll want to inspect their sarcophagus. Once you've got both pieces of Kangaxx return to him in the Docks District and prepare yourself for an extremely hard fight...
The fight with Kangaxx will be extremely difficult, arguably more difficult than the final boss battle of this game. I'd recommend you check out my BG2 Beginner's Guide if you need tips/tricks and strategies for fighting difficult foes like Kangaxx.
Kangaxx also has immunities to at least 3 dozen different status effects and types of damage. For a complete list you should check out Kangaxx's wiki page - his list of immunities is far too long to repeat here. A way you can make this fight easier is to use Protection from Undead on everyone in your party except one character. You can buy Protection from Undead scrolls from many places, check the wiki for a complete list.
I'd recommend you wait until at least Chapter 6 before challenging Kangaxx due to the difficulty of the encounter. One thing I didn't mention yet is that the Lich part of the Kangaxx fight is just the first half!
When Kangaxx turns into his Demilich form he'll become significantly more powerful. Kangaxx will gain a 90% resistance to all Physical Damage and will be immune to all weapons with less than a +4 Enchantment. All of his Saving Throws will also be increased (except for Save vs Spells) which will become worse.
There are 2 spells that Kangaxx will use in his Demilich form that are extremely annoying, Imprisonment and Demilich Howl. Both of these spells will give you an instant game over if they hit your main character which in my experience makes this battle mostly about RNG. Demilich Howl you can protect against by using Death Ward but Imprisonment there is nothing you can do against.
Due to Imprisonment being so overpowered this mostly comes down to a numbers game in my experience. I typically try to ensure that I have 5 Summons active during this fight to give Kangaxx more targets to choose from. If any character other than your main character gets hit with Imprisonment it's no big deal since you can use Freedom to free them right after the battle. However your main character getting hit = instant game over.
If you'd like a detailed strategy for dealing with this version of Kangaxx, again, I recommend you check out the Kangaxx Strategy page on the wiki. That page lists all weapons that can hit Kangaxx along with various other useful tidbits of information that can make this fight easier.
Your reward for beating Kangaxx will be the Ring of Gaxx which is arguably the best ring in the entire game.