Number 6 Lightning and Destruction Style turns almost his entire body
into plasma. Not including some of his skeleton and brain. By itself plasma cannot be physically touched anymore than gas or air can be touched.
Knuckleduster even acknowledges that Number 6 can't be damaged by attacking the plasma.
And when Koichi hits him, we see his attacks don't actually land on his body and instead disperse his body .
The only reason Koichi is fine with throw these attacks at Number 6 is because he knows it won't kill him.
"
Elemental Intangibility: The ability to become intangible by transforming into a substance, such as wind, fire, or water, making it so that most attacks merely disperse them instead of truly damaging them, allowing them to regenerate. However, this would still leave the user vulnerable to many forms of attack, such as sufficiently powerful energy or elemental attacks."