The Showdown

Fear the Octo Samurai is the second boss mission in Splatoon 2's single-player mode, Octo Canyon.

Walkthrough
In order to kill the Octo Samurai, you must splat it to make it drop its splat roller on the ground. The roller grows into a larger form and a tentacle pops out of it. Attack the tentacle 3 times and you win.

First Phase
The boss will fling its splat roller in a similar fashion to inklings, but has a very wide range. The Samurai can also get on top of his weapon and charge towards the player, leaving behind a wide trail of ink. If he gets a direct his, its an instant Splat. The boss takes a break every time he does this, letting the player get in free hits. He always does a backward jump before doing his charge attack.

Second Phase
The Octo Samurai does everything he does from phase 1, but he has two new attacks:
 * He jumps towards the player and does a vertical swing with splat roller.
 * He quickly spins around with his roller leaving a circle formation of ink. A direct hit is an instant splat.

Third Phase
Most of his attacks get more vicious, and he has a new one:
 * The Octo Samurai performs three, extremely long ranged vertical swings.
 * He chases you when he does his charge attack. He still takes a break after this.
 * He will always swing three times in a row when he does his regular swing.
 * He always follow up his jump attack with three vertical swings. These swings are not has long ranged as his other vertical swing attack.

Boss demonstration
https://www.youtube.com/watch?v=1mhvoYbr0QE