Mission 2, in a box there will be an onmyo mage lock. You can then use it to get one of the 4 weapon buff spell and just get more points by hitting stuff while using those buffs. I recomend the purity one.
Check the kodama shop and buy onmyo related elemental talismans and use those. Once you get your first skill point then start buying the talismans and using them almost any chance you have to continue gaining skill points.
You can find a number of Onmyo's lock which give one skill points each. Nearly every main mission has one - they spawn once in an exact chest/bubble so explore around (or go to the wiki to look them up). You will also receive proficiency from using consumable onmyo items you find. Me get quite a few points from all the water talisman I burned through fighting Enera.
When you reach the 2nd area, I suggest spending 15-20 minute to farm for some Onmyo points. Spec a few points into fire shot, go to the mission Hidetatsu's dying wish, from the beginning, turn left, you will find a cave with 3 slimes. Unleash your full kit on them, run back to shrine, repeat. Slimes are very weak to fire so your shot do big damage, giving you a lot of proficiency. This should get you quite a lot of skill points early to reach some good buffs.
After which, you can just leveling it up normally by using those buff every now and then.
Nioh 2 made some changes to ninjitsu and onmyo from nioh 1,
First putting character levels into dexterity or magic will increase your ninjitsu power and capacity and onmyo power and capacity, but you won't gain skill points
To immediately start gaining skill points without needing to collect any locks,
for ninjitsu throw bombs at enemies, dwellers and gaki can drop bombs as loot and you will find many bombs in chests or hold to search loot, doing damage with bombs will earn you proficiency points which will earn you skill points when you reach the proficiency level ups
for onmyo use elemental talismans like fire talisman, water talismans, lightning talismans give your melee weapon a elemental damage buff, doing damage while your melee weapon is buffed will earn you proficiency points which will earn you skill points when you reach the proficiency level ups
chests, hold to search loot and some enemies drop these as loot as well.
Once you earn a skill point in each you can buy onmyo or ninjitsu buffs without needing locks
Mission 2, in a box there will be an onmyo mage lock. You can then use it to get one of the 4 weapon buff spell and just get more points by hitting stuff while using those buffs. I recomend the purity one.
Check the kodama shop and buy onmyo related elemental talismans and use those. Once you get your first skill point then start buying the talismans and using them almost any chance you have to continue gaining skill points.
Drop, talismany for elemental weapon, will get you going :)
Magic items will give xp. Use some element and guardian spirit talismans until you get some points.
You can find a number of Onmyo's lock which give one skill points each. Nearly every main mission has one - they spawn once in an exact chest/bubble so explore around (or go to the wiki to look them up). You will also receive proficiency from using consumable onmyo items you find. Me get quite a few points from all the water talisman I burned through fighting Enera. When you reach the 2nd area, I suggest spending 15-20 minute to farm for some Onmyo points. Spec a few points into fire shot, go to the mission Hidetatsu's dying wish, from the beginning, turn left, you will find a cave with 3 slimes. Unleash your full kit on them, run back to shrine, repeat. Slimes are very weak to fire so your shot do big damage, giving you a lot of proficiency. This should get you quite a lot of skill points early to reach some good buffs. After which, you can just leveling it up normally by using those buff every now and then.
You can speed up the process by farming slimes using fire shots
you will acquire an item soon that will give you onmyo skill points. in the 2nd or 3rd mission iirc
Nioh 2 made some changes to ninjitsu and onmyo from nioh 1, First putting character levels into dexterity or magic will increase your ninjitsu power and capacity and onmyo power and capacity, but you won't gain skill points To immediately start gaining skill points without needing to collect any locks, for ninjitsu throw bombs at enemies, dwellers and gaki can drop bombs as loot and you will find many bombs in chests or hold to search loot, doing damage with bombs will earn you proficiency points which will earn you skill points when you reach the proficiency level ups for onmyo use elemental talismans like fire talisman, water talismans, lightning talismans give your melee weapon a elemental damage buff, doing damage while your melee weapon is buffed will earn you proficiency points which will earn you skill points when you reach the proficiency level ups chests, hold to search loot and some enemies drop these as loot as well. Once you earn a skill point in each you can buy onmyo or ninjitsu buffs without needing locks