Heres a scenario:
An English knight of the 15th century travels to Japan. Being royalty, he is made a guest of one of the Oyabun, which insults a visiting samurai duelist. The knoght is challenged to a duel to the death for the honor of the Oyabun. If the knight refused, he is branded a coward and killed.
The knight having spent some time in Japan has some idea of the martial arts of the place. Being a foreigner he is allowed to choose his weapons and armor. Normal japanese duels are fought with no armor.
The knight appears in a helm, full chainmail, breastplate, gloves and a broadsword.
The Samurai seeing the knight wisely opts to wear his armor of wood lacquered breastplate, heavy leather gloves and light chainmail. with Katana. He does not wear a helmet to show his contempt.
Using western swordfighting techniques which uses every part of the broadsword from its armor peircing sharp tip, to its bludgeoning hilt. The samurai is taken aback by this technique because to the untrained eye, it almost seems the knight was wildly swinging his weapon all around him. His slashing cuts are accurate and wouldve cut thru most japanese armor but the knights thicker chainmail and good defense of the apparent weak spots does nothing to hurt the knight.
The knights blows however are staggering. Even a parry was painful and only the superb craftsmanship of the samurais katana keeps it from breaking. It suffers huge gouges in its sharp edge from trying to stop the heavy english sword. A slash downward strike to the neck from the samurai is captured in the metal gloved hand of the knight. Holding fast to the katana, the knight twirls his other arm on top of the katana and puts his whole weight on the sword, the knight disarms the samurai and kills him.
Ya dont need plate armor to fight man to man. Plate armor was invented to stop arrows and a small mob of untrained villagers with small swords, A sensible knight would not go to a battle wearing that thing. You only wear as much armor as you need to to protect yourself from the waepons of your opponent. If all he has is a slashing sword, then chainmail is quite appropriate.
Samurai were pitted weapon against weapon. armor was a sign of cowardice in a one on one duel. Thats a disadvantange against a man who wears armor most of his waking day. The skill with which an english knight can bring on to a battle would surprise and confuse a samurai who have never seen a knight fight on foot. The average samurai warrior will fall to an average knight. Why? because a knight does not fight with a style that uses a countermove for every move. A knight is used to cutting down opponents both skilled and unskilled. A samurai would not know how to properly counteract an unknown skill.