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 his peak potential, which would have taken decades to realize and was able to reach his Awakening, thus increasing his magic power so that he was able to still use Ittou Shura despite one minute already passing and had enough to perform Ittou Rasetsu soon after. Training with Edelweiss, Ikki had attained better control of his magic resulting in him being able to improve his Ittou Shura, but it is not yet known by how much.

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 proved to be a match against her using Dragon Spirit.

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.
 * During his training with Edelweiss, Ikki theorized that he should be able to make Ittou Shura as strong as Ittou Rasetsu.