Complete all previous Rehearsal Room modes, then complete Ultra 
mode in the Rehearsal Room as the indicated character as an enemy
to unlock the corresponding bonus troop type.

Iron Man Archer  : Archer as the enemy.
Iron Man Knight  : Knight as the enemy.
Iron Man Mage    : Mage as the enemy.
Iron Man Monk    : Monk as the enemy.
Iron Man Soldier : Soldier as the enemy.