WoWDB

Kor’kron Warbringers are the adds that spawn in phase 1 of this encounter.  Typically they are cleaved down by the DPS and finished off by an Iron Star, the large fiery wheels that roll across from the edges of the room.  All 18 of the adds must be killed by the same Iron Stars.

To complete this achievement, you will need to hold off DPS until you have built up 18+ Warbringers.  This will be a lot of damage on the add tank, so cooldowns may be required.  Get all the adds to low health (~20%) to ensure the star wil kill them, and make sure they are hit and killed.

Once you have successfully killed all 18 with a single Iron Star, you complete the encounter like normal.  Credit will not be granted until the boss is downed.

Currently bugged. I did LFR Wing 4 "Downfall" tonight and all my group received this achievement. Another person on the official forum said that his non-LFR group got it even though he knew that they hadn't killed the requisite number.

This was corrected in an October 29th hotfix.