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But I'm On Your Side (25 player)
Defeat the Assembly of Iron while under the effect of an Iron Boot Flask in 25-player mode.
But I'm On Your Side (25 player) requires you to defeat all three bosses of the Assembly of Iron in Ulduar's 25-player mode while transformed into an Iron Dwarf via the 
Iron Boot Flask.
Unlock the Flask Vendor
Before you can purchase the Iron Boot Flask, you must first 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 from Olut Alegut (Horde) or Rork Sharpchin (Alliance) in Storm Peaks for 10 Relics of Ulduar. As of patch 6.1, the flask is a toy rather than a consumable, and it transforms you into an Iron Dwarf for 10 minutes with a 1-hour cooldown.
Use Flask Before Final Kill
Enter Ulduar and defeat Steelbreaker, Runemaster Molgeim, and Stormcaller Brundir. You only need to be in Iron Dwarf form when the last of the three bosses dies, so activate the flask immediately before the killing blow to avoid wasting the 1-hour cooldown. Note that Druid shapeshifting and Warlock Metamorphosis will both cancel the flask's transformation.
Tips
Druids must remain in their default form for the flask transformation to stay active — any shapeshift will cancel the Iron Dwarf effect.
Warlocks should avoid using Metamorphosis while the flask is active, as it will cancel the transformation.