Im guessing he let his enemy touch the doll and crush the doll by creating a vacuum? Also what @azsy said is also possible but i doubt the author would go for such an easy win lol
@Zreth Basically he was walking around with a wooden sword and scabbard the entire time. When he needed to quick-slice someone he used his power on the exposed piece of wood sticking out of the scabbard. This turned the inside of the scabbard and the wood "blade" into metal, giving him a near friction-less surface area to draw the blade from and slash with full speed.
@Zreth If I understood right, the guy's sword which orginially is wood can cut through anything but since he uses a sword style that relies on unsheathing the blade in a quick motion, forgot the name, he has to activate the ability while the sword is sheathed. So I assume if he doesn't have the wood inside the scabbard the blade will cut through the scabbard as well everytime he tries to sheath it to do another attack resulting in him cutting himself accidentally?
I suspect his ability modifies the optical density of air rather than its physical density, otherwise it would make much more sense to attack enemies directly. It would also explain why he thinks his ability pairs particularly well with hers.
@KageLycan Although the thing I think the author doesn't know is that if you're doing Iaijutsu properly then there should be no contact between the blade and sheath while drawing it anyway meaning that if the guy actually knew what he was doing he wouldn't need a trick like that.