Duryodhan's armour was built by Drona during 14th day when Arjun was creating havoc in war. It was some special Mantra chanted armour, nobody could have pierced it except Arjun. He pierced Duryodhan twice on 14th day by targeting the spaces left between to joints of armours.
There was no way to pierce it using Mace(Gada), that's why Duryodhan chose mace fight with Bheem so that he couldn't be hurt by Bhimsen. There is no doubt that both Bhim and Duryidhan were best Mace fighters and with and advantage of armour it was impossible to beat Duryodhan.
So Krishna told Arjun that and Arjun reminded Bheem his vow. That Gandhari's boon to Duryodhan is just a folklore..
