The Slave Pens is in Coilfang Reservoir in Zangarmash. Ahune is only in the Slave Pens during the Midsummer festival. You will need the quest Ahune is here! given by Numa Cloudsister who's right inside the instance. She will send you to Luma Skymother who is just past the first boss. Luma will then give you the Summon Ahune quest. Bring the provided totem to the Ice Stone to summon him. Here's how the fight will work:

Phase 1

Ahune himself does not need to be tanked as he has ice around him that will continue to stun players. During Phase 1 he will spawn an elite that the tank should pick up and be killed ASAP. The elite has an aura that causes frost damage every few seconds. Along with the elite he will spawn a group of non-elites. These have relatively low health (around 2k) and can be burned down easily by the DPS. Ahune also shoots a spike out of the ground every so often. There will be a radius on the ground, so move away from it to avoid being thrown into the air.

Phase 2

Ahune will now retreat and become more vulnerable to damage. Here's where you want to really give him all you've got! Pop your trinkets and cooldowns during this phase.

After Phase 2 he will repeat Phase 1 and so on and so forth until he is dead. Each Phase cycle could bring more difficulty as your group could wind up with a lot of little adds. When you kill him, make sure to loot the Shards of Ahune. Bring them back to Luma Skymother to receive either Tabard of Summer Skies or Tabard of Summer Flames and complete the achievement. You will also receive 20 Burning Blossoms. The loot from the boss comes from his Ice Chest, so don't forget to loot it!