I'm currently running a Sword-focused build with minimal Ninjutsu and a lot of Magic + Spirit built into it, and a few of the things I made sure to buy with Ninjutsu points were Sneak attack, Catwalking and Suppa. Imagine my surprise though, once I got the sword's back attack mystic art and stacked back attack bonuses with the Kingo set, I found out that Iai Strike – if silenced by Catwalking since it alerts the enemy without that – does about twice as much as Sneak Attack when used from behind. Without buffs my Iai Strike can one-shot almost anything, and even stronger demons tend to survive at only 10-30% HP. If I stacked Power Pills and/or Carnage Talismans I could probably one shot pretty much anything, making this the ideal stealth assassin build as far as I can tell.
I haven't gone whole hog with it yet because I was more focused on parry-comboing humans and doing big damage by dodging behind demons, but I don't see anything else on the skill trees that could do this stealth thing better. Has anyone found anything else that fits this archetype? Even if it's not as good, I'm curious how Kusarigama Ninja, for instance, take advantage of stealth. I'm also curious how the axe's charge attacks compare, since although they also do heavy single hit damage they don't get the boost of the sword's "damage from behind" mystic art.