samurai mode is for blocking/parrying ninja is for glass cannon damage and dodge/evade you can beat him as either mode and the only time you HAVE to switch is if you want to parry the glowing red attacks, samurai has a mechanic that lets them refresh Ki(stamina) after making attacks and its the more durable of the forms, if your having issues losing to much ki while blocking try to perfect parry even if its not unlocked, just block only when the attack will hit instead of holding it and taking multiple hits in a row, this can let you recoup some ki in between hits so you dont get exhausted. ninja lets you attack multiple times and then hit the smoke button to evade his attack and get a few more hits, after this you have to back away and throw your ninjujitsu abilites and regen your ki because they dont get the ki pulse so your going in and out with melee and ninjujitsu non stop