The fragments are only generated during Phase 1 of the fight, when Bronjahm can be kited around to prevent the Fragments from reaching him. However, on entering Phase 2, Bronjahm teleports to the center and remains unmovable.
Earning this achievement relies on the fact that the Fragments can be slowed, ensnared, etc. Kite Bronjahm to ensure fragments don't reach him, and ensure that the fragments are (or will be) slowed or immobilized away from the center of the combat area. Don't start phase 2 (by reducing him to about 30% health) until you have all four fragments up and usefully positioned. When Bronjahm teleports to the center, burn him down as fast as possible.
- Patch 3.3.0 (2009-12-08): Added.