How to Beat Orochi
You can brute force it by taking damage upgrade passives, upgrading your equipment to +5, and putting Samidare 8 on your attack. You don’t need to worry about elements if you do it this way.
At the blacksmith. Whenever you have him work on an item, there is a chance to increase the level of it.
Items with No Passive have a high chance to increase an item’s level (and won’t mess up your build by changing the current passive). This makes items without a passive some of the best drops you can get.
When equipped as armor, upgraded items give bonus defense and speed. When equipped as a weapon or sub weapon, upgraded items give bonus damage.
Slash Down + Samidare + Expand. Just mash as many of those 3 into a single attack as you can. No need to bother with elements, combos, or anything else. It will explode Orochi heads in a few swings each.
The only thing you need to do re: equipment is get a piece of Samurai Soul on the weapon type you’re using then make sure that weapon type is equipped in every slot. That alone is enough to kill just about anything in the game in a single Samidare 8 swing. I spent ~4 hours collecting and upgrading a full set of maxed out gear (I thought you had to level it up by combining it with the same skill, lol) before the first time I killed him but it was a big waste of time because you’re going to trivialize him off of Samurai Soul anyway, regardless of your equipment upgrade levels.