The Orb Whisperer (25 player)

Source
The is the first encounter in the Crimson Hall wing of Icecrown Citadel.

Strategy
One of the easier achievements, proper management of the Dark Nuclei (ensuring that Keleseth's tank gets them) and Conjure Empowered Flame (ensuring that there are enough people to get between the ball and the target to reduce the damage) are key.