Complete any level with Iron and 2 other skulls activated on Heroic or Legendary
This will be familiar to anybody thatís used the skulls in the past Halo titles. As everybody knows, Iron causes the level to reset if you die solo or automatically revert to a checkpoint if one player dies in co-op. I highly recommend doing this achievement in co-op (at least a dummy controller) to eliminate frustration.
This doesn't have to been done with Iron on, at least not in the Euro version. The achievement just said to finish a level on Heroic or above with ANY 3 skulls on.