I think I've just found my definitive samurai builds for the weekly challenges as they are now.
The first one is for facing the hordes of onis on nightmare survival. With this technique setting and gear, my samurai has 5 resolve slots and fills it a lot by taking damage. I can switch to all sword stances to face different enemies. I usually approach a group with flamable dirt throw to stun them, turn on explosive blade and way of the flame and hack away. When they fight back, I take the damage to fill my resolve quickly, then I finish with one or two Hachiman's Fury in a row (when the five slots are filled, I have one to go and one 2/3 filled, making it fast to strike twice in a row). I figured since you're bound to take a beating at nightmare mode, I could at least take something from it. Cannot win in a group without at least one ronin (then again, who can?).
The strenght of this build is how all around consistent I could craft the gear's buffs to be, so that almost everything is stacking. I have two +25% Injured Resolve Gain stacked, but I don't really know how the game processes it. Does it become a +50% bonus or is it 25% and then 25% over that? The result was good, anyway. Crow lancers power me up really fast!
This is Beat-Me-Up Samurai:
Techniques: Explosive Blade, Defender, Resolve Increase and Legendary
Gear: Master's Katana L, 110 ki (all four instances, Melee resolve gain +10%, Perfect Parry Window +12%, Way of the Flame), Half Bow E, 110 ki (Ranged resolve gain +10%, Draw Speed+12%, Headshot Refund), Last Breath L, 106 ki (Instead of taking fatal damage, heal for 50 life --this can happen once every 5 minutes, Injured resolve gain +25%, Resolve gain +10%, Resolve increase), Dirt Throw E, 110 ki (Perfect Parry Window +10%, Cooldown Reduction +14%, Fired Up), Healing Gourd E, 110 ki (Damage Reduction -9,7%, Injured Resolve Gain +25%, Strong Brew).
For the Tale of Iyo Chapter III, in which you have to face her in a one on one duel, I made a different build, focusing on Perfect Parry Window. This one worked perfectly, I could easily parry and open Iyo's guard every time she made a regular or blue shine attack (which is all the time, so if anyone was having difficulty with her, this is one way to have better odds at the duel).
This is Suck-It-Iyo Samurai:
Techniques: Explosive Blade, Defender, Critical Defense, Legendary
Gear: Stone Striker L, 105 Ki (Heavenly Strike, Perfect Parry Window +12%, Counter Damage +20%, Stone Master), Half Bow E, 110 ki (same), Samurai Charm E, 110 ki (Perfect Parry Window +12%, Counter Damage +20%), Lady Sanjo's Surprise L, 110 ki (Dirt Throw has 15% chance to cause enemies to allucinate, Perfect Parry Window +12%, Damage Increase +20%, Fired Up), Caltrops E, 110 ki (Damage Increase +20%, Melee Stagger Damage +12%).