Guide to Defeat Tutorial Boss
Assault mode is a scam. Use quick thrust to boost THROUGH/PAST the missles and gain ground on the boss that way.
Then when in range, wait for a volley, boost through and double sword slash the boss’s nose.
If you are firing your missles whenever they are available, eventually the boss will stance break.
Make sure when he does, you are in range for a sword assault. Try and make reads or guess where the boss will fly to next and beat it to that spot with a sword slash.
This all helped me beat it.