All my characters kill Kazuro on their first runs of Shiroda Fortress (As compared to my Hard Warrior who re-runs a zone a few times for some equipment before taking on the boss with the exception of Shiroda which is also cleared on the first time.)
Even if the equipment is for better or worse, useless, unless you're a Premium player and those equipment cost quite the massive dollar, some people do like to collect the equivalent to the ultimate equipment for that time, it keeps them occupied, and it makes re-runs to torment that smarmy git for his Crystals easier.
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On a side note, I completed my Hard Warrior run, estimated amount of time taken, 6 hours, 1 hour slightly longer than Normal Assassin and Priest run.
Takeshi went down quick,
Asura required me to switch to using Bladestorm instead of Doomblade,
Tetsuo taught me to time Shockwave to weaken his Soul Shatter from 2K to 300,
And Kasumi failed to understand (Again.) that Tank Warriors counter Samurais and Assassins.
The last two bosses were piecemeal, the 2 Shiroda Priests might as well have been the big bosses, considering their Soul Shatter deals 4.1K DMG (And I have maxed out Heavy Armor and blocked it when I couldn't run.) as compared to Shiori's 3.3K eye. That's saying something.