#querks

Ground rules: i am using information from the anime ONLY if anything i say is contradicted in the manga don't spoil anything. I will be using information and observations from the last episode of season 3 and the first arc of season 4. if you have ideas of your own i can't stop you from sharing them in fact please do, and if you feel the need to tell my im an idiot whos overthinking things please do.

with that out the way let's look at the facts:

Mirio can “permeate” (pass through) solid matter, poor translation really. 

but, to the point how? here's what i think, Mirio clearly still has mass as gravity still affects him, now atoms are 99% empty space and are constantly vibrating, so, if he were able to change the speed at which his molecules vibrate he could make his molecules slip past the atoms of other objects while still being subject to forces 

this would explain why he can do it with individual body parts as the atoms are still bounded together even if some of them are moving at a different speed. 

it would explain why his costume (which is made of his hair) can permeate as well, as it is still capable of changing vibration speed.            

it would even explain why it takes so much effort to use as if he has the ability to change the speed which his molecules vibrate rather then having the ability to switch them from solid to not solid, he would have to concentrate on keeping them within the margin of error for being able to pass through.

so how would you defeat him: as he is still subject to forces, so a telekinetic or similar should be able to imopalize him. Electrocution should still work.

my personal strategy would be to use electrofied caltrops as the bottoms of his feet still need to be solid stand in the middle in a rubber suit and when he attacks activate the electric caltrops (high voltage electric arcs would be best) so, if he touches (or in the case of electric arcs goes near them) he will be shocked give it enough power to incapacitate or kill a fit healthy human male and bam one deep fried Togata