Any of the three Dark Realms can be easily bypassed. This requires some skill, Ako's Purple Vest, all weapons for the character, and all weapons been completely enhanced. Whether or not the gauntlet is completely enhanced is irrelevant. Before entering the Dark Realm, equip either Chigo (for Samanosuke) or Hyosai (for Jacques). The enemies that attack will appear in multiple waves. Just wait for all enemies to appear, then execute one magic attack. Absorb all the souls that appear. Ako's Purple Vest should allow you to do this as many times as necessary. However, if you are fighting large demons, you may need to get up close with either the Tenso (for Samanosuke) or the Enja (for Jacques). Note: A crucial part of this strategy is absorbing the souls. If you feel that you need Ako's Blue Vest as well, make sure to switch back and forth as appropriate. Also, it is much easier to go straight for the bottom part of the Dark Realm that you are in, as the tougher enemies are down there first. By doing this, you can use your magic efficiently on the tougher enemies and not have to worry about the weaker ones.
Defeating Guard Dog
Note: This was done under the normal difficulty setting. First, stock up on magic for all your weapons. Then, stock up on all the Purple Souls. At the beginning, jump behind him after he attacks. Then, just waste the magic on him. Use every weapons magic until you do not have any more. After that, use regular attacks. Watch out for the ice attack. Most of the time the thunder magic from him is off target. After a few regular attacks, transform into the Onimusha and attack him hard. However, do not use the mega attack yet. After a few attacks, just before your magic runs out use the mega magic attack on him. If he is not defeated, then just attack him a few more times.
Defeating Nobunaga
When you are fighting Nobunaga when he is in his demon form, press Square(2), Triangle. This is a great combo move, but do not overdo it with him. When you absorb the darkness from his sword and you get it, remember that some of his moves cannot be blocked (such as the charge and the fireball moves). You can only block one or two. For the charge move, press Triangle at the last moment when he is on fire and you should dodge it. For the fireball move, just have enough space to run around.
Defeating Two Marquless
When playing as Michelle, go further behind near the skull door and throw some grenades on them.
Deflecting critical hits
Repeatedly tap Guard until someone hits you. And as soon as you see the flash, tap Square. Keep tapping Square to go from enemy to enemy, racking up multiple critical hits.
Devilish mode
Successfully complete the game in Critical (Issen) mode.
Easy Mode
Get "Game Over" twice.
Hard Mode
Complete the game on normal mode.
Heihachi Buraiden Mini Game
Finish the game.
Issen Mode
Complete game on hard mode.
Jacques' alternate costume
Successfully complete the game under the normal difficulty setting with a high score as Jacques. Alternately, successfully complete the game as Samanosuke's alternate costume.
Kill all Genma onscreen
This can be done without charging during an Onimusha. Also, block Issen works during an Onimusha and takes out all enemies.
Michelle's Costume
Complete the game with Onimusha ranking.
Michelle's second alternate costume
Successfully complete the game with an "Onimusha" rank. Alternately, play the game with a memory card with a saved game from Onimusha Blade Warriors.
Mini Games
Oni Target Practice: Complete the game on a normal setting to unlock oni target practice. Character must use guns or arrows to pick off Genma eggs that go by on tracks.
Adventures of Heihachi: Complete the game on a normal setting to unlock this short storyline featuring Heihachi.
Oni Training: Complete every training session and beat the game to unlock this bonus training session.
Genma Puzzle Space: Solve every Puzzle Treasure Box in the game and complete the game to unlock this mini-game.
Oni Mode
Complete Issen mode.
Original Onimusha: Warlords swords (Japanese version)
Successfully complete the game with all three items from the Phantom Realms. Then, start a new game to have all three swords from the original Onimusha: Warlords.
Alternately, play the game with a memory card with a saved game from Onimusha: Warlords,
Panda Costume
Complete Secret Training mode.
Panda Suit Photo Opportunity
In the Bolougne Zoo, just outside of the building with the air-sealed door, there is a wooden photo shoot prop. Stand behind it and the camera shows your character in the panda suit from Onimusha Warlords, with Ako as the baby panda.
Play as Gargant
Gargant can become a playable character in Onimusha: Blade Warriors. Have a saved game with a clear status on a memory card in slot 1 when booting up Blade Warriors.
Puzzle Mini Game
Complete all puzzles in the normal game.
Quick Health
In the Zoo, Stand behind the panda cutout and equip the white vest on Ako and watch your health go up quickly.
Ranking Mode
Complete the game on normal difficulty. Two ranking screens will be available. The first one displays the individual scores for each character while the second displays an overall letter grade and any earned titles.
Equip Ako's White Haori then stand still. When Ako's wearing the White Haori, rotate the Right Analog-stick . Ako will move faster to regain health quicker, depending how fast you can rotate. Note: Do not not use it while you have five Oni Orbs.
Samanosuke’s Warlords Weapons
To unlock the three weapons that Samanosuke wielded in Onimusha: Warlords obtain the Fire Seal, the Thunder Seal and the Wind Seal and complete the game. The next time the game is played through the three weapons reappear in the alleyway where Samanosuke materializes in the present time period.
Samanosuke's alternate costume
Successfully complete the game under the normal difficulty setting with a high score as Samanosuke. Alternately, successfully complete the game as Jacques' alternate costume.
Secret Training Mode
Complete normal training mode, and complete the game.
Shooting Mini Game
Complete the game on Normal difficulty.
Skip FMV sequences
Successfully complete the game. When playing the game again, press Start to advance past any intermission sequence.
The 4th 'Dark Realm'
I know it may not sound real but it is true! There is a 4th Dark Realm.
The 4th Dark Realm is inside the Oni Mansion. Once you are inside the mansion reach the end of the maze.(Beside the protective mirror.) Hit with any of your weapons the pink screen. Enter the small passageway where on your left is a hole and on your right is a bundle of boxes.
DO NOT ENTER THE HOLE!
Keep pressing the X button on the boxes. After whenabout 2-5 seconds the man, wrapped in the green material who hangs from the ceiling, shall appear. If you press the X button on him he shall ask you if you would like to go to the Dark Realm.
Note:
1. I have only tried this on Hard Mode.
2. The reward should help you in the next battle!
3. Obviously the Oni Mansion is only available when playing as Jacques.
Ultimate Mode
Complete the shooting mini game.
Unlimited Souls
To acquire an unlimited amount of souls, after obtaining the Oni Army Orb, get to second area. Kill off initial wave of foes, and find something to hold down the soul absorption button. Your Oni army will continue to defeat the spawning Genma, and you will receive all their souls. This will take some time before acquiring a large amount.
Unlock Ako's Black Haori
Successfully complete Issen/Oni training.
Unlock alternate ending sequence
Increase all seven Haori of Ako to the maximum. This requires all Kodama to be found.
Unlock Bishamon sword
Successfully complete the game with the "Onimusha" rank. Alternately, get the Bishamon sword in the Phantom Realm in Honnouji when Sam goes to the past.
Unlock Oni Musou
Have Jacque go through the Phantom Realm in Honnouji.
Unlock the Three Swords of Onimusha
Complete all of the phantim realms, finish the game, and start new game.