Generally u want to maximize either the hp stat or tai/nin, depending on ninja role. Also decreasing the lowest useful stat: def on ninjas, that are backline or tai/nin, if any ninja doesnt use it.

Roshi +hp, -atk

asuma +nin -def

iruka +atk(dmg) +res (shield) -def

darui +nin -def

bee dont have him, but propably same as iruka, since he fulfills same role

roshi is a bit special, since u for 100% want to max his survivability and scaling def/nin can make up for it if you want to decrease tai dmg stat

\/ thx, that's what I was missing with roshi. I dont have him so couldnt check growth rates.

Roshi +HP, -atk for me. I relies on other stuff he has scaling nin and does more NIn than atk anyway.

For Roshi, I do +hp and -nin. I do this because, he gets back that nin he lost, when his passive activates over and over again.

Roshi's passive works like crimson fist passive. The higher the basis is the higher the mutliplier let it become. More or less when your rivals on par power have a 4k nuke ready for you, you have just a 2k one in reply. dAnd you seem to forget another importat factor where ninjutsu comes in play: self healing. While you heal 1000 your rival that selected ninjutsu heals 1600, every round.

That's why I did -atk. All of his kits are nin base more than atk. Rather not lost 5% then make it up.

Why not just lose 5% on atk and dont need to make it up for later?

Yes, currently -atk is the best solution. Since what you want is for him to endure the most possible to stack up a ton you can't lower resistance, defense or hp, so you have to reduce one between atk and ninjutsu, but ninjutsu is way more useful for him then that's why you select attack. About which one to boost among the 3 you don't absolutely want to reduce the choice could be defense or hp, but for how currently defense scales up and considering that most of the attacks are nin/tai or full nin then hp is a better solution since works always.