No, you should do +hp -def because he will stack defense regardless and his basic defense is pretty poor so you don't lose much or +hp/-atk but this one is less appealing.

If you don't mood hp roshi he just simply flat out dies like in arena so there's not much to choose.

Doing -nin makes no sense.

You are indeed right there, but how much do you lose? Defense is one of his lowest stat by default and is also, generally, the lowest stat you gain from equipment. That -5% means at most 100-200 less defense that could mean maybe 1% damage reduction agaist full tai attack (that are not many to tell the truth), so what is, hm... 50 real damage less on the first attack that hits him?

Do -def.

Should be +12% nin +4% def per hit endured.

-Atk works indeed if you aren't betting a lot on the damage his mystery and standard attack land.

Remember that generally your enemies own a ton of resistance and pretty poor defense. So that -atk sometimes could hurt a lot, overall against enemies like swordbearers team with root of warrior.

None did it because all the major treats to roshi are full ninjutsu or half nin/half tai effects. If you mood -res your roshi get owned by a gaara or by an ay 3rd of a player even 10k less powered than you if he owns initiative.

Remember that roshi usually gets unwillingly hit by standard attacks that generally aren't that strong except rare cases, but if gets target directly is by full nin or half nin/half tai mysteries.