The Midsummer Festival quest to kill Frostlord Ahune can be quiet epic for a five players group in heroic mode depending on the gear and skills of each player. In order to start the quest chain, head to Shattrath City by the Midsummer Festival area located north of the flightmaster.
Follow the links below to read an article with all the quests involved to summon Frostlord Ahune. A video of the fight in heroic mode below. Make sure to bring mana oil and flasks. It is a tough fight: