Ittou Shura

"One minute was enough. It was fine to ignore what happened afterward, but for one minute he'd become strong enough to beat anyone."

Ittou Shura (一刀修羅,  One-Blade Shura) is the Noble Art of Ikki Kurogane.

Description
Ittou Shura is the Noble Art, in which Ikki is able to amass all his bodies vigor and stamina while ignoring the survival instincts that prevent humans from using their full strength, then multiplies this several dozen times granting him tremendous power, which can be maintained for just one minute and as such, it is referred to as his strongest minute.

During his match against Touka Toudou in the finals of the Selection Battle Arc, in order to defeat Raikiri, Ikki had used an improved version of Ittou Shura, Ittou Rasetsu to defeat her.

In the Seven Stars Sword Art Festival Finals, as Ikki fought against Stella using Dragon Spirit, he had reached the peak of his potential, which would've taken decades for him to realize and reached his Awakening not wanting her to find another rival, further increasing his magic power to where he could still use Ittou Shura despite one minute passing and had enough to perform Ittou Rasetsu soon after.

In the Vermillion Empire Arc, after training with Edelweiss, Ikki had attained better control of his magic resulting in him being able to improve his Ittou Shura, despite it not being shown, Edelweiss had stated that Ikki could make it almost three times as strong. And Ikki theorized that he should be able to make his natural Ittou Shura as strong as his Ittou Rasetsu.

Weaknesses
Ittou Shura expands and drains all of his stamina and strength for a minute, so Ikki will be unable to fight using it for an extended period of time. Its usage is limited to once per day due to the incredible strain that it puts on him and has visited the hospital a number of times after using it, while at times having enough strength to make it to his room. This is a weakness that a number of opponents have taken advantage of as shown by Ayase Ayatsuji who wanted to cripple his chances of winning.

Though the power granted by Ittou Shura is tremendous, it doesn't grant him a guaranteed victory. It is first shown in his initial fight against the Twin Wings and World's Strongest Swordsman, Edelweiss who proved too strong for him to match leaving him on the defensive. In his fight against Stella Vermillion in the finals, he only managed to match her using Dragon Spirit but was not able to outclass her.

Through his training with Edelweiss in order to improve his magic control, he was able to overcome some of the limits of Ittou Shura and improve it, but by how much was unknown.

Trivia

 * The Shishou martial arts technique, Kirinkou, is very similar to Ittou Shura.