List of all Onimusha 2 achievements:

Light Souls

Absorbed demon souls for the first time. Obtain automatically as the game progresses.

Lightning Strikes

Obtained the Buraitou. Obtain automatically as the game progresses.

Piercing Cold

Obtained the Hyoujin-Yari. Obtain automatically as the game progresses.

Slashing Gale

Obtained the Senpumaru. Obtain automatically as the game progresses.

Tremor Hammer

Obtained the Dokoutsui. Obtain automatically as the game progresses.

Running Flames

Obtained the Rekka-Ken. Obtain automatically if you follow this guide. Require completing Purple Phantom Realm.

Brains Over Brawn (Secret)

Defeated Ginghamphatts. Obtain automatically as the game progresses.

Now Is Not The Time (Secret)

Crossed swords with Gogandantess. The easiest way to do this is to use Lightning Sword and keep attacking him until you both cross sword. Restart if needed.

Flexing in the Forest (Secret)

Defeated Ginghamphatts, again. Obtain automatically as the game progresses.

Avenge Your Mother (Secret)

Defeated Jujudormah. Obtain automatically as the game progresses.

Still Untouchable (Secret)

Fought Gogandantess a second time. Obtain automatically as the game progresses.

The Greatest Swordsman of All Demons (Secret)

Defeated Gogandantess. Obtain automatically as the game progresses.

Keeper of the Gold Scale (Secret)

Defeated the Soul Queen. Obtain automatically as the game progresses.

Third Time's the Charm (Secret)

Defeated Ginghamphatts, but for real this time. Obtain automatically as the game progresses.

Nobunaga's Perdition (Secret)

Defeated Nobunaga Oda, the Demon King. Obtain automatically as the game progresses.

Jubei's Fate (Secret)

Defeated the Golden Evil Statue. Obtain automatically as the game progresses.

Good Memory!

Put the three Nobunaga statues in their correct positions. Obtain automatically as the game progresses.

Blowing the Lid Off

Entered the Infernal Cauldron. Obtain automatically as the game progresses.

No Turning Back

Entered the demon base. Obtain automatically as the game progresses.

Nice Place You Got Here

Completed your first Phantom Realm. Obtain automatically if you follow this guide.

Glad That's Over

Completed all the Phantom Realms. Obtain automatically if you're following this guide either on "Kotaro Route" or "S-Rank Route".

No Help Needed

Completed the game on Normal or higher without enhancing weapons or using Jewels. This can be hard. However, you can cheat by completing it in "Ultimate Mode".

Ultimate Demon

Completed the game in Ultimate Mode.

Sorry, Coming Through!

Completed the game in less than 5 hours.

Pro-nimusha

Completed the game and got an S Rank. I have a separate section for this above.

Super Slider

Completed the Puzzle Phantom Realm.

Stylish Samurai

Completed The Man in Black.

Gathering Forces

Completed Team Oni.

Tokichiro's Mistake

Obtained the Great Powder Tube. Obtain automatically as the game progresses.

Sharpshooter

Obtained all long-range weapons. Obtain automatically if you follow this guide. If you can only unlock this in Magoichi Route. Alternatively, you can play with "Ultimate Mode" on; then the achievement will be automatically unlocked.

Walking Library

Collected all files. Obtain automatically if you follow this guide.

Map Maniac

Collected all maps. Obtain automatically if you follow this guide.

Two Balls Short of a Dragon

Obtained all five Oni orbs. Obtain automatically as the game progresses.

Legendary Warrior

Unlocked Jubei's alternate outfit "Fashionable Goods 2". Obtain automatically if you follow this guide. On the 20th floor of the Purple Phantom Realm, put the Rekka-Ken back into the cracked floor to open the 21st secret floor.

Fashion Model

Unlocked Oyu's alternate outfit "70's Set 2". You have to complete 100% scenarios, which means that you have to complete "Ekei Route", "Magoichi Route", "Kotaro Route", and "Oyu Route".

Pour Your Soul Out 

Enhanced equipment at a Magic Mirror for the first time.

Check out My Collection!

Enhanced all enhanceable weapons to maximum. You have to collect enough red souls to upgrade all weapons.

Armor Aficionado

Enhanced all enhanceable armor to the maximum. You have to collect enough red souls to upgrade all armors. Alternatively, you can play in "Normal Mode" and turn on "Ultimate Mode"; as soon as you appear, the achievement is automatically unlocked. "Ultimate Mode" starts the game with armor fully upgraded.

Top Condition

Used Power Jewels to increase your health to the maximum. Obtain automatically if you follow this guide. Only if you choose "Full Trade" or "Essential Trade" option.

Massive Mana Pool

Used Magic Jewels to increase your Magic Power to maximum. Obtain automatically if you follow this guide. Only if you choose "Full Trade" or "Essential Trade" option.

Gone in a Flash

Used a Critical to kill a demon. Try practicing the demon soldiers, they are the easiest enemies to perform the critical hit.

Pushing Back

Won a power struggle with a demon. Try to attack at the same time when an enemy is attacking you; well, this really depends on your luck.

Heir to the Oni

Transformed into an Onimusha. Press L2 + R2 at the same time when your Onimusha gauge is full.

The Final Battle (Secret)

Transformed into a true Onimusha. Obtain automatically as the game progresses.

Human Abacus

Solved all number puzzle boxes. Obtain automatically if you follow this guide.

King of Diamonds

Solved all tablet puzzle boxes. Obtain automatically as the game progresses.

Treasure Trove

Opened all of the puzzle boxes. Obtain automatically if you follow this guide. Only if you choose "Full Trade" or "Essential Trade" option.

Dig Dog (Secret)

Obtained a Gold Coin. Obtain automatically if you follow this guide. Basically, you have to pet the dog in front of the gold mine entrance 5 times (stand face to face with the dog and press "O"), then follow it to pick up the gold coin where it's digging.

Who Needs Rods? (Secret)

Caught a river fish. Obtain automatically if you follow this guide. The river fish appears in the river at Fog Valley.

Don't Even Need a Bigger Boat (Secret)

Absorbed souls from a shark. Obtain automatically if you follow this guide. After you unlock the Scarlet-key door, you'll notice a shark tank on your left. When the shark swims near you, press and hold "O" to absorb souls from the shark.

A Token of Friendship

Gave an item to one of your allies. Obtain automatically if you follow this guide.

Beware of Mushrooms

Gave unique mushrooms to one of your allies. Obtain automatically if you follow this guide.

Forgive and Forget

Give a Wonder Flower to one of your allies. Obtain automatically if you follow this guide.

Trusty Teach

Got an A rank during Oyu's fencing lessons. Obtain automatically if you follow this guide, Oyu Route.

Serious Synergy

Got a combo when giving items to an ally. Obtain automatically if you follow this guide. Or simply give Comb + Mirror to Oyu.

The True Onimusha

Collected all Honors.