The Iron Knuckle is not only a sub-boss, and it's found in almost every zelda game.
The way to kill them it's diferent acording to the game you are playing.
In the legend of zelda, just avoid them, and keep swinging your sword until it's defeated.
In The adventure of Link, keep thrusting up and down, trying to avoid it's shield, and after a few blows it's done.
In Ocarina of time, avoid the axe and hit until it's death.
In Majora's Mask, do the same from above.
In The wind waker, there isn't Iron Nuckle, but exist an enemy that's basically the same, it's a Dark Nut, and to defeat them, just wait, aiming at him, until you are able to make a parry attack, and after two of this (One up, and one side), He'll be without armor and then it's easily defeated.
In the Minish cap, It's also a Dark Nut, just avoid his attacks, and then attack him by the side.
I don't recall if there is another Iron Knuckle in any other zelda game, so if you know of anything, edit this text.
N.B.
I am at a loss to understand why the armoured enemies in The Twilight Princess are almost universally referred to as Darknuts when they clearly have more in common with Iron Knuckle.
To my mind, the distinguishing features of Darknuts are that the armour contains a dog or jackal headed warrior and is more efficiently attacked from the rear. The armour itself consists of two parts, the helmet and breastplate, which must be removed separately in order to attack. Conversely, the Iron Knuckle armour is usually empty, can be attacked from almost any angle, and sheds a multitude of parts with each attack until only the inner chain mail remains. As is mentioned in the article, an Iron Knuckle is also considerably more powerful than a Darknut. While it is true that Knuckles in previous 3D incarnations of the series have carried axes rather than swords, it is also true that in their very first appearance during The Adventure of Link, this was not the case. Therefore the difference in weapons does not distinguish between the two as the article suggests.
If any in-game evidence exists to say that the warriors in question are indeed Darknuts rather than Iron Knuckles, then I am happy to go along with that, although I feel that it would be wise to include this evidence into the article in order to dispel confusion between the two separate enemies.