Kun is physically the stongest here due to smashing that thick wall of Mando Iron. Especially when you consider that the Dark Lady forged the handle of her lightwhip out of the near-indestructible Mandalorian iron, and uses that hilt to block opposing lightsabers - such as Vaders. Then one must remember that little incident with Luke Skywalker, where the Grandmaster confronts the person who (he thought) killed his wife, and decides to batter his opponent with signature Form V attacks:
One-two-three-four; she blocked him, handle held this way, then that, using the whip like a short lightsaber to deflect him, but Luke didn't pause or shift direction to wrong-foot her. He drove her like a battering ram toward the edge of the mesa, pushing her within meters, then a step, of the edge. Lumiya held the whip handle in both hands like a staff and blocked his downward sweep. For a moment they were locked in a stalemate, pushing against each other and grunting with the effort, with only the sounds of exertion because they had nothing left to say to each other.
Sacrifice
Lumiya - while driven back to a cliff edge - still manages to parry every single one of the Jedi Grandmaster's strikes with the hilt of her whip. A hilt made of the same material that Kun - not close to his prime - destroys in two hits.
Originally posted by MythLord
Kun wasn't even physically smashing the Mando Iron. From what I recall, he's increased the intensity of the lightsaber's cutting power and then proceeded to cut through it.
Customized controls allowed him to adjust each blade's length from half a meter to one and a half meters. Panels controlled each blade's intensity: at its highest setting, a blade was like any other lightsaber, with the ability to cut through dense materials such as armor plating and durasteel with virtually no resistance. At the lowest power setting, a blade became a simple shaft of light, delivering no damage and providing no resistance against incoming lightsaber attacks.
Excerpt - The Essential Guide to Weapons and Technology
The only two people who are stronger are Anakin (zone) and Luke. But given Kun's Force feats, it's no surprise his augmentation is ridiculously above - pretty much everyone bar those two. Caedus couldn't break Beskar with his strength or his lightsaber, and needed to learn how to exploit the flaws in Mandolorians' Forging process with shatterpoint.
Oddly enough, Kun's lightsaber cuts through the iron without much issue after he sets it to it's highest intensity. Also noteworthy is how the same scan that shows him accomplishing the feat notes he senses a power that "wants to help him" as he sets the blade to it's "highest intensity" and then breaks the wall:
To answer your point, AP, the wall wasn't that thick, seems only like a foot or so: