Hello to the only 2 to 8 people actually playing this game and welcome to my 100% guide!

Why making a guide on a game that not many people plays? Well, because there is some misinformation on both guides already available; And don't get me wrong! Both guides actually helped a lot, but I needed to investigate further more to actually unlock some achievements, so that's why I wanted to make a guide as clear as possible. Basically I'm just merging both guides already available + adding my own experience. I had some issues so you don't have any.

REMEMBER: ACHIEVEMENTS DON'T POP WHEN YOU ACHIEVE THEM LIKE OTHER GAMES. When you get an achievement for this game, you need to exit the game and then it'll pop.

Estimated time to platinum: Around 15 to 25 hours. It depends on your skill level and If you're actually enjoying the story or just skipping everything.

The Wonders of Width
Completed Asuka's story.
You can unlock this achievement regardless of difficulty.

For the Safety of this World
Completed Ikaruga's story.
You can unlock this achievement regardless of difficulty.

Queen of the Hooters Harem
Completed Katsuragi's story.
You can unlock this achievement regardless of difficulty.

Love for Hibari!
Completed Yagyu's story.
You can unlock this achievement regardless of difficulty.

Sweets Go Somewhere Else
Completed Hibari's story.
You can unlock this achievement regardless of difficulty.

Comedy Gold
Completed Homura's story.
You can unlock this achievement regardless of difficulty.

The Sprouts Forgive You
Completed Yomi's story.
You can unlock this achievement regardless of difficulty.

This is My Calling
Completed Hikage's story.
You can unlock this achievement regardless of difficulty.

Am I an Adult Now?
Completed Mirai's story.
You can unlock this achievement regardless of difficulty.

Let's XXXXX!
Completed Haruka's story.
You can unlock this achievement regardless of difficulty.

It's Aww for Juthtisthe...!
Completed Yumi's story.
You can unlock this achievement regardless of difficulty.

The Path of the Artist
Completed Murakumo's story.
You can unlock this achievement regardless of difficulty.

Feel Free to Concede
Completed Yozakura's story.
You can unlock this achievement regardless of difficulty.

The Story of my Tragedy
Completed Shiki's story.
You can unlock this achievement regardless of difficulty.

I Cook Happiness!
Completed Minori's story.
You can unlock this achievement regardless of difficulty.

Nobody Calls Me Handsome
Completed Miyabi's story.
You can unlock this achievement regardless of difficulty.

Level Up my Shut-in Lifestyle
Completed Murasaki's story.
You can unlock this achievement regardless of difficulty.

All Right! You Pass!
Completed Imu's story.
You can unlock this achievement regardless of difficulty.

Awakening to Maternity
Completed Ryobi's story.
You can unlock this achievement regardless of difficulty.

Do Not Try This at Home
Completed Ryona's story.
You can unlock this achievement regardless of difficulty.

Girls' True Faces
Completed Story Mode with all the default Hanzo x Crimson Squad characters.
Once you completed all 20 achievements above, you'll unlock this achievement too.

For some reason, even achievements that specify "Hanzo x Crimson Squad characters" actually means every single character in the game besides Daidōji and Rin. I've read on other discussions that since this version of the game included all DLCs, every character is a default character now, so that's why you need to do all characters even when the achievement specifies the Hanzo x Crimson Squad only. That doesn't explain why Daidōji and Rin aren't needed tho. I just know when the achievement pops and now you do too!

While completing the story (or your first 100 battles) you should also get all these achievements!
And If you don't, don't worry! You'll have 100 battles left to earn them, so keep calm.

Super Dish Initiate
Completed any mode once.
Just complete a battle in Story Mode, Arcade Mode or Free Mode.

This is Just the Beginning
Reached Ninja Arts Lv. 5.
While your Ninja Art is activated don't miss a note and It'll level up. Just reach Lv. 5.

Absolutely Perfect
Completed a cooking battle without any mistakes.
Yes, you can do it! If you're struggling, try on Easy and you'll get it!

Have a Rich Experience!
Reached a total of 5000000 (5 million) points.
Depending on your performance, you'll be granted with XP at the end of a battle. Unlocked naturally.

Not a Beginner Anymore
Hit a total of 5000 notes.
One of your first milestones. Unlocked naturally.

Nice Boobs
Hit a total of 11081 notes.
Why 11081? Because it means "いいおっぱい" = Nice Boobs, lol. Unlocked naturally.

Unstoppable Woman
Used a total of 100 Ninja Arts.
Only activation counts and not leveling up. Activate as soon as possible! Unlocked naturally.

A Repertoire of 100
Completed 100 songs!
Just complete 100 battles within Story Mode, Arcade Mode or Free Mode.

In order to see a Special, you have to complete the first and second section of a song with almost no mistakes (one mistake per section is usually fine). If done correctly, you'll receive half a heart on each section making a full heart. Then, in third phase you have to hit the heart note that'll appear (pressing any key) and If you complete the song, you'll completely strip your opponent before seeing "Perfect Victory" on your screen. When you achieve a Perfect Victory, you've seen a Special.
You can also check how many Specials you've seen on Main Menu -> Library -> Record.

Special Delivery
Made your first Special.
Strip your opponent completely for the first time.

Breathtaking View!
Saw 50 Specials.
Strip your opponent completely 50 times. Try on Easy if you're struggling.

Harem of a Hundred
Saw 100 Specials.
Strip your opponent completely 100 times. Try on Easy if you're struggling.

A Lot of Boobs
Saw all the default Hanzo x Crimson Squad characters' Specials.
Strip all characters (excluding Daidōji and Rin) while playing in Any Mode.

Let me elaborate a little more. You need to see the special of every character (excluding Daidōji and Rin) while playing Story Mode, Arcade Mode or Free Mode since this achievement is cumulative across all modes. Sadly, there is no possible way to see witch Special you're missing, so you need to rely on your memory (as you should've stripped a lot of characters while playing the Story or other game modes). If you know you haven't stripped a specific character, just battle against that character in Free Mode on Easy. If you don't know which character is left to strip, battle each of them on Free Mode on Easy at least once and you should get the achievement.

Isn't weird to see a Dressing Room achievement with less than 10% having it? I've read that is bugged, but it's not! Actually, you only need to do one thing, but it's very specific.

Shy Girl
Played in the dressing room.
Zoom completely to any character's breasts until she says something. That's it.

Sexual Harassment King
Fully enjoyed the dressing room.
Zoom completely to every character's breasts until they say something. You're done.

Once you've reached your first 100 battles the pain begins as now the only thing left to earn all achievements is grind, grind and grind. (Not that much tho)

Oh My...
Lost in a cooking battle and got stripped.
Just AFK a battle.I know this is not a grind, but I didn't know where to add this achievement...

Finally!
Reached Ninja Arts Lv. 15.
Easiest way to achieve this is against Miyabi on Normal. Do your best and aim only for perfect hits!

This Woman Knows Stuff!!
Reached a total of 10000000 (10 million) points.
If you want to achieve this before your 200th battle, play on Normal (songs grant more XP).

Happy Cooking
Hit a total of 50000 notes.
If you want to achieve this before your 200th battle, play on Normal (songs have a lot more notes).

Master of Ninja Arts
Used a total of 500 Ninja Arts.
Activate as soon as possible and fail next note on purpose to activate multiple times in a single battle.

Panties Collector
Obtained all 49 panties.
Complete every character Story Mode on all 3 difficulties.

Luckily for us, there's a cheat code (from publisher) for this achievement, so no grind!
Just go to the Main Menu (where you select game modes) and input as fast as you can:

• while holding LB + RB (Xbox Controller) simultaneously press ↑←→↓↑→←↓↑→↓←↑↑←↓→↑.
  
• while holding Q + E (Keyboard) simultaneously press W A D S W D A S W D S A W W A S D W.

If you can't get the code to work, just change your Settings to 640x360 Windowed and 30 FPS.

Cooking is My Specialty!
Completed 200 songs!
We avoided grinding for too many achievements. This'll be your final achievement! Have fun!

Thank You for Cooking!!
Obtained all trophies.
SWEET VICTORY!