Honestly with most ninjas I chose Ninjutsu.

His growth is on Ninjutsu so go for that. He hits like a truck

i used tai and got better results than nin

Resistance for pos 2-4 (better shields and tankier)

because i used a 2x atk with him.there's a 4.5k dmg dif if you use tai and you lose only around 2k on mistery from what i noticed

Rather Simple 2,3,4 don't really do to much damage unless your a spreader (which only really works if your a mega whale, running around with 30k sub stats in all position easy way to lose against someone with a 40k pos 1, but anyways)
position 2,3,4 just need to survive
Why res over def?
1. Less Ninja's do Tai heavy damage and more of a nin favored side
2. Hes shielding is base off 20%-40% of his resistance, higher resistance higher shields meaning your more tanky on everyone.

I don't really see a reason for putting position 2-3-4 on attack/nin (There more useful tanky and less damaging then damaging but fragile) , I personally just do +65% Resistance and depending on there growths (I.E Kimi H) I would run +65% Defense, Quite rare for me to do +65% Life.

Hope the answer good enough on at-least how I view things.