-( Story related, cannot be missed )-
There are three stages to this battle.
First Stage: You will need to intoxicate Orochi using the 8 Purification Sake pools. Stick to the first three heads (I took on Fire, Curse, and Wind). Avoid Fire's fire streams and, when it goes to roar, Waterspout the Sake into its mouth. Repeat this once more to intoxicate the head, then proceed on to the next. (Proceeding are if you follow the suggestions and take on the other two heads mentioned) Next up is Curse, which will shoot two cursed buds at you and then roar. Bloom the buds to prevent the curse from hurting you and make the head drunk. The last one will attempt to hit you with a cyclone, then roar. You might want to take this one or the next one, Water, as both are very simple to dodge. Dodge Wind's cyclone and then get it drunk, or wait for Water to flood the arena with water and attempt to rest its head in the water itself. When its head is down, hit it to aggravate it and it will roar.
By the time you've made three heads drunk, the entire body will be inebriated, exposing the Prayer Bell on top. Climb up and destroy the bell to remove Orochi's invulnerability and proceed into the second stage.
This time, you have to get each head drunk only once, but once drunk, it will fall to the ground and be vulnerable to hits. You have to repeat this all eight times (once per head) and while they may use their signature attacks, chances are they'll just lash at you. Wait for them to roar, feed them the Sake, and then attack them.
Susano will now step up to the plate to take credit for the entire thing. Once Amaterasu clears the sky and he holds his blade up high, draw a moon in the sky to start the mini-battle off right. For every slash that Susano deals towards the snake, make sure you follow immediately after with Power Slash. If you fail any of the seven entries, you will have to repeat the entire thing. Susano will take care of the eighth on his own (good for him!).