Successfully complete the "One Man Army vs. AIM-X" challenge, 
  which is unlocked after Story mode mission AIM-X, where you must destroy four 
  power couplings before any are repaired, then defeat Horgan and his giant 
  cannon. Hulkbuster is slow but can sustain massive amounts of damage. Its 
  Repulsors are more like cannon shots, and in place of grenades and missiles is 
  a single cannon shot that hits a target, then a moment later creates an 
  explosive shockwave. It is perfect for groups of enemies.