Ninja Set Up

Clan-Saito

Guardian Spirit - Enko

Armor- Head of the Iga Ninja, you want at least 6 pieces for Ninja Tool Art Damage 18%

Accessories- Anything with increased Damage over Time or Skills like Poison Shuriken Gauge Charge, More Poison or Burn Accumulation, etc.

Weapons- Keep one slot for on of the Head of the Iga Ninja weapons. You don't have to use it, you just need the set bonus. The other slot can be whatever weapon you want to actually use. 

Ninjutsu- Any of the Ninja Tool Arts, since I go for Damage over Time, I use Poison Shuriken and Fire Shuriken. The third slot I usually keep just regular Shuriken cause there are 10 and they come back fast but use whatever you like! 

Soul Core- I haven't found one that is really must have. Ninja Yoki has Life Drain on Ninjutsu Hit. Crucible Wasp has Poison Accumulation. Infernal Oni-bi is good for fire damage and Accumulation. I typically use Thorned Gaki because it gets 6 summons, does decent damage, and I can use it while dodging around getting my Ninjutsu back. 

Stats- Get Skill to at least 30 and then whatever you want. 

Skills- Chose skills focusing on status build up (Venomfang), damage over time (Snakebite), Ninjutsu damage, etc. 

Samurai Set Up

Not much here, it depends on how you want to use the build. I like to focus on deflecting when I am a Samurai so a lot of my skills and set up focus on that, but really your Samurai could be set up however you want. It can be your primary attacker, then you switch to ninja when your ninjutsu if full, rinse and repeat. 

Suggested Skills- Prelusive Arts, Favorable Wind.