Plate armour is explicitly designed to stop sword slashes. A sword blade has little kinetic energy behind it compared to other weapons, and it certainly cannot slice through metal. The best way to deal with a man in plate armour is in one of two ways: if facing him with a sword, grab the sword halfway along the blade, known as "half-swording", to obtain greater tip control and use that to thrust through small gaps in the opponent's armour. Grappling and wrestling to force the opponent on to his back so a dagger can be forced through his armour's gaps might also be used. Alternatively, seek to cause blunt-force trauma with a weapon like a war hammer, an axe, or a flanged mace, which make use of heavy heads to concentrate a huge amount of kinetic energy into a point. Even if that does not crack armour, it will send a huge amount of energy reverberating through the opponent's body, doing him injury, nd that's also going to smash a shield to pieces.