Ikki activates Ittou Shura
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.
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 already passing and had enough to perform Ittou Rasetsu soon after.
In the Vermillion Empire Arc, training with Edelweiss, Ikki attained better magic control, improving his Ittou Shura, despite not being shown, the former had stated that he could make it three times as strong as before. Ikki, on the other hand, theorized that he might be able to regulate Ittou Shura in combat, while the magic that would end up being dispersed can be applied to Intetsu. If he were to achieve this, then his Ittou Shura would evolve, making it as powerful as Ittou Rasetsu.
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.