But I'm On Your Side (10 player)

Requires the player to be under the effect of the Iron Boot Flask when the last of the tree Iron Council bosses is killed. The flask can be purchased in Storm Peaks from Olut Alegut (H) or Rork Sharpchin (A) for 10 Relics of Ulduar. You will first have to hand in an aditional 10 Relics of Ulduar (H) / (A) to unlock this vendor.

The Iron Boot Flask in a non-consumable item that turns the player into an Iron Dwarf for 10 minutes, with a 1 hour cooldown. The player only has to be in Iron Dwarf form when the last boss is killed, so saving the flask for just before the killing blow seems like a sensible approach to prevent wasting the cooldown or being forced to stay in caster form as a druid.

This is going to be a pain for druids (particularly ferals), since shapeshifting breaks the costume - its this achievement all over again.

I have just done this achievement, and i can confirm you will get credit for this achievement if you pop the flask when the last NPC is at a low percentage.