Here's a quote from chapter 4 of the LN:
“Yeeart!”
“Yeeart!” Ninja Slayer’s suriken attacks cannot breakthrough the muteki attitude defense.
“Yeeart!”
“Yeeart!” The suriken get rejected one after the other.
“Yeeart!”
“Yeeart!” At this rate, he’ll make no progress. Realizing this, Fujikido Kenji made a new move.
“Yeeart!” First, he threw a suriken.
“Yeeart!” Iron Vise turned his body to steel again.
“Yeeart!” Then he threw another suriken.
“Yeeart!” Iron Vise defended himself in the same manner again.
But while turned to steel, Iron Vise cannot move for one instant. So if that’s the case, how about throwing suriken even faster?
The first teachings of Ninja Slayer’s instructor Master Dragon Gendoso-sensei echoed in the neurons of his brain.
“Fight fire with fire? One cannot. Make up your mind to fight fire with speed and even deadlier speed you shall create. If the enemy doesn’t fall after a hundred suriken, throw a thousand.”
Ninja Slayer swung both his arms alternately around like a hellish pitching machine throwing a suriken every zero point five seconds.
“Yeeart!” Throw!
“Yeeart!” Muteki attitude defense.
“Yeeart!” Throw!
“Yeeart!” Muteki attitude defense.
“Yeeart!” Throw!
“Yeeart!” Muteki attitude defense.
“Yeeart!” Throw!
“Yeeart?” Muteki attitude defense.