I find the variable durability over the years of both the armor and the Mandarin interesting. Sometimes the Mandarin seems to have superhuman durability and sometimes he doesn't. Here he fails to damage the armor with martial arts strikes, yet he manages to later karate-chop off chunks of armors several iterations more advanced, or dent them with kicks. My best guess is that his physical abilities are dependent on his supply of chi. He begins fights with a full supply of chi, and has superhuman strength and durability as a result. As fights go on, he runs dry, and eventually ends up at human strength and durability.