at the beginning of the fight 4-jump shuffle to charge your boost, strike him with your hits (i call it the shotgun) then he shouldhit the platform before spinning -> charge shot 4 -> jump shuffle ->shotgun, then you should be overdrive lvl 2.

When he spins & bounce : try to connect all hits of the shotgun each bounce+ a few hits, try to switch at the last moment to win a few hit. before he destroy the platform with the last bounce charge shot him, & 4 jump shuffle.
You should be OD lvl 3 normally

The rise: 2 seconds where everything is played. He hits the platform from under you: you MUST do shotgun + charge shot at the frame he hits the platform and you get near OD lvl 4.
Then 4 jump shuffle, and KEEP IT he will come from the right : shotgun.

The true shotgun :
Go on the platfomrm underneath him and shotgun his legs as fast as you can, when he is almost close to you charge shot and he will go back then restart the shotgun, you should kill him.

The approximate time is 24 seconds, so its pretty intense, i m actually on the 20th rank of time attack : SAMURAI, so if you have any questions ask me and i ll be pleased to answer you
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This game is just awesome, make it live and expand !!!
here is the video of the fastest time i ve found: ( i'm not playing i don t have the rights of this video etc..)
https://www.youtube.com/watch?v=pOjaU8ykrZQ