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But I'm On Your Side (10 player)
Defeat the Assembly of Iron while under the effect of an Iron Boot Flask in 10-player mode.
But I'm On Your Side (10-player) requires you to defeat the Assembly of Iron in Ulduar's 10-player mode while under the effect of an 
Iron Boot Flask when the last of the three Iron Council bosses dies.
Unlock the Flask Vendor
Before you can purchase the Iron Boot Flask, you must hand in 10 Relics of Ulduar to unlock the vendor — Olut Alegut for Horde players or Rork Sharpchin for Alliance players, both located in Storm Peaks.
Purchase the Iron Boot Flask
Once the vendor is unlocked, purchase the Iron Boot Flask for 10 Relics of Ulduar from Olut Alegut (Horde) or Rork Sharpchin (Alliance) in Storm Peaks. The flask is a non-consumable toy item that transforms you into an Iron Dwarf for 10 minutes, with a 1-hour cooldown.
Use Flask Before Last Kill
Enter Ulduar and defeat the Assembly of Iron. You do not need to be in Iron Dwarf form for the entire encounter — activate the Iron Boot Flask when the last boss reaches around 2% health to ensure you are transformed when the killing blow lands, preserving the cooldown for the rest of the fight.
Tips
Druids should avoid shapeshifting after activating the Iron Boot Flask, as shapeshifting will cancel the Iron Dwarf transformation and may cause you to miss credit for the achievement.