This build is the build I completed the game (NG) with and found it pretty easy to annihilate everything. This isn't the best build in the game though. It is just what I finished my blind playthrough in NG with.
Summary:
*CORE STATS*
Heart, Skill and Magic were my focus. Heart gets the most skill points by far.
Other stats are leveled up as required to unlock whatever extra samurai/ninjutsu/magic perk I was chasing and to make armor set perks work.
*ARMOR*
I played the game with whatever was the best level set I could get until the Red Demon Armor set dropped. Once it did, it was all I used - refashioned into the Iga Ninja set of course.
I always tried to spec for DEFENSE/LIFE/TOUGHNESS/KI related perks. DAMAGE FROM BEHIND is also a priority if it rolls. But as I didn't farm gold at any point, upgrading gear was too costly so I went for higher level armor over perks if the level was a lot higher.
*WEAPON*
Katana with +FIRE. Used the Hikoemon Ichimonji for most of the game, which has +FIRE by default.
Spear (Red Demon Armor set one). Since getting this is rare unless you farm, I pretty much spent a lot of my soul matching on upgrading this to keep it leveled up as I went.
Gfor CLOSE COMBAT ATTACK perks. I use low stance a lot so low attack perks work for me. Mid attack are also good and have plenty of moments where they are great.
Choose PARRY if you intend to parry a lot. In your sword skill tree, couple this with parry skills that turn your opponent around, or that drop them to the ground. Since I go for damage from behind perks, parrying adds to your damage output ability.
Choose BREAK for breaking the guard. Choose the one that relates to your preferred stance. For me, that was low stance mostly, so LOW BREAK. If you like low stance too, also look for QUICK ATTACK DAMAGE.
*EQUIPMENT*
I mixed this up a lot, but basically went for LIVING WEAPON ATTACK ENHANCEMENT and LIVING WEAPON DURABILITY a lot in the beginning. The better I got and the less I relied on living weapon I changed to RECURRENT DAMAGE FROM FIRE and FIRE DAMAGE as this supports the Red Demon Armor set well. Also, YOKAI CLOSE COMBAT DAMAGE and VS YOKAI REALM help deal big damage to tough opponents and bosses.
*GUARDIAN SPIRIT*
Enko as it pairs with Red Demon Armor. However, most of the game I used the first one Kato as it's also pretty good and Enko comes later.
*CLAN*
Li. It gives you Red Demon Armor close combat attack and toughness perks. It's only 2% and 8% but any increase is a good increase.
*SKILLS*
Samurai - Skills that boost Ki are a must, including using Ki pulses when dodging. I put that on all stances as I didn't want to accidentally be in a stance I wasn't away of and suddenly finding out my dodge has left me without Ki, which happened at the start to me a lot. Living weapon (QUADRISECT) is a must I think. Choose whatever other skills you like, but I like the mid stance parry Water Shadow and Water shadow II. Get your parry right and you can destroy opponents when you spec for large damage from behind as the parry turns their back to you.
Ninjutsu - I put a point into Ninja Tool Mastery to save on putting points into Dexterity. Max out the passive SNAKEBITE TECHNIQUE for increased damage. SNEAK ATTACK is also a must, and is a killer with increased damage from behind perks. I also max out my arrow capacity with QUIVERMAKER.
Magic - Weakness Talisman, Carnage Talisman, Sloth Talisman. I maxed these out so I get the choice of how many I want to carry at any one time. Max out PURE MIND to get max Yokai Realm passive damage. Also take EVIL WARD which is the opposite of PURE
MIND - reduces damage from Yokai. Put a point into PROTECTION and STEEL talismans for tanking yourself up - very handy with light armor. Also maxed out Living Weapon time with the INANIMATE ENCHANTMENT ability.
With this build you run alot, so Ki pulse all the time. You also get into close combat constantly, so low attacks are common. If you're behind an enemy switch to mid stance to deal more damage faster. If you max out all your damage from behind perks, pop a carnage talisman and a weakness talisman, then sloth an enemy so you can just stand behind them, you'll annihilate anything that's tough in seconds. Some of my boss kills take 10-20 seconds at most, with most of that time spent getting behind them, while the damage to kill takes the least amount of that time.
Enjoy!