After 3 wipes I suddenly facerolled it.
I ran in circles backwards with my shamans on follow. Dropped all the usual totems and kept renewing the magma totems when they expired. Every time I dragged all the adds past the fire totems, I would hit fire nova. Once Ahune retreated to his frozen core I'd hit a spread-out macro on my shamans so that they would break follow and face Ahune. My pally would then run up to the frozen core and DPS the core along with the shamans. Together with the bloodlust, I did about 6-8kl DPS per shaman so I got him down in 1 core-phase.

After that it was loot and done.