I haven't done this quest yet - I just got back from a wedding and am getting caught up.
But, I hope to tackle this today:
I've quoted this from WoWHead.com, it was written by Femaleshammy -Al'akir -eu server.
Full guide of "How to" (complete Ahune the Frost Lord)
-First. Go to any major city(Exodar, Darnassus, Thunder Bluff, Silvermoon City, Ironforge, Orgrimmar, Undercity or Stormwind) talk to "Earthen Ring Elder"
-Go to the camp with alot of Twilight people, kill people there untill you get a letter, use a quest item(A totem of some sort), which is provided in this quest, near one of the bonfires and summon your Shaman. Speak to him to get the next part of the quest.
-Head abit north, to the western "cap" that peaks out to the water, into a new twilight camp, use the disguises you now got in your bag, and turn into a crab, wait near some friendly NPC(They are hostile without the disguises) untill your quest is complete(hope no other player is killing the npc's) and go to nearest bonfire, summon the shaman, deliver in.
-Head back to the guy you started this chain at, and you will now get the FUN quest. Which is one daily quests, including killing a guy near the Dark Portal, (which I'm not going further in at) and a quest that takes you to Slave Pens.
-Find a proper 5 man group, go into slave pens, deliver the quest at the lady you will see with some totems. Kill some packs of mobs into the next room and next NPC.
-ALL PARTY members go deliver, and when ready, one guy alone, will go to the ice shard, and deliver in, makes you summon the boss, abit further in, about the place where you normally would kill the "crab" looking guys, so remember to clear abit first.
-THE BOSS FIGHT-
-PHASE 1- The boss at first will have a armor that you can't remove, that reduces damage by 90% or something like that, now you should focus on adds. All rangers, and the tank, should kill the big "ice elemental" that has a model like a earth elemental, which meleers kill the smaller water elementals. (the big guy gives a debuff to nearby players that takes 300damage per third second, and stacks untill he's dead) This phase last for about a minute.
-PHASE 2- Now all remaining adds should be down, and the healer can get his mana back up, you won't leave combat, but all that is left of the boss now, is "Frozen Core" which doesn't do any damage, but you are now free to damage without any reduction.
These two Phases repeats untill the boss is down, if you are good, you can take him down on two "rounds of phases" a decent group should on 3, and if you use 4, you should be out of mana.
The boss is now down, and you will loot a chest that spawns in front of him, it gives the epic loot(caster staff, healing cloak, tanking cloak or melee cloak) along with a shard, that gives a quest, you accept this quest, turn it in to the lady not far from you, and get a tabard(you can choose between red or blue).
If there is several people in your party the long chain quest, they can also summon the boss, and you can kill him as man times as you want to, without resetting the instance.
Enjoy the Midsummer event.
-Femaleshammy -Al'akir -eu server