Does his passive stack up with Wind Dance and Red Lotus Secret? Because I'm fairy certain it doesn't with Warrior's Killing Intent (it does stack with Anbu Special Tactics).

Thank you for those line-ups, I'll be sure to test them (at least the ones I'm able to).

Yes, but there are sword users in game (Anbu Itachi, Shisui etc.).

I tested this with LM's Warrior's Killing Intent. There was no difference in LM's standard attack damage with Warrior's Killing Intent active. His damage went up when I switched to Anbu Special Tactics. So it's incorrect to say that oly elemental enhacement doesn't stack.

It won't be as good with water, because water has no skill to buff his mystery. Maybe he can fit in some teams as a support.