I think the key to defeating people in power armor isn't puncturing the armor, but finding a way to exploit whatever weak spot it has or the person controlling it has. In WW2, Russian infantry was often ill-equipped to deal with German tanks, but at times we able to improvise ways of disabling them even if they would never actually puncture the armor. The molotov cocktail for example was useful as way of attacking the crew inside the tanks with fire, by having flammable liquid pass through the slits used for vision or breathing. Other options included destroying the treads or even prying open the hatches with crowbars. Obviously these tactics cost a great deal of human life, but were all things that could work eventually. In dealing with power armored troops you'd want to find similar weaknesses to exploit. Examples could be the power supplies, the breathing apparatus, blinding attacks, destroying their weapons. Even things like crude pit traps could be effective at immobilizing heavily armored troops.