You will need two playthroughs to 100% Shiness, I'd suggest playing through it once without too much worry, and then finish up what you missed on your second playthrough. There are 2 endings.
Either you fight Irys, or you fight Kaorys. You also have to beat the game without getting a game over, and get an S rank on any of the major bosses. See "Some like it fat".

There is only one place which you cannot return to which is "The 3 Arrows", which makes Serkan Gear Solid, and Another World achievements missable.

3 quests are also missable if you progress too far without having finished them,

• A Healthy Stroll
  
• A spy on Mantara
  
• Koclibabs

Every achievement can be gotten via multiple save files, so if you miss anything on the first playthrough, you can get what you missed, and still unlock the achievement(s).

The Fallen King
Defeat Meos.


The Matriarch
Defeat Mother.


Iron Man
Break free of Ayron.


Dream Team
Have 5 characters in your team.


Rumbling thunder
Defeat Irys.
To fight Irys you have to do the following choices while playing through the game:

• When you meet Skulfon, give him his arm back.
  
• After beating Grorim, spare him.
  
• Help Zlog the Explorer.
  
• In Manatara City, warn Kaorys.
  
• In Giant's Hall, help Nullakah Jones.
  
• During the "A Spy in Mantara" side-quest, accuse the Children.

Rampant betrayal
Defeat Kaorys.
To fight Kaorys you have to do the following choices while playing through the game:

• When you meet Skulfon, beat him up.
  
• After beating Grorim, kill him.
  
• Kill Zlog the Explorer.
  
• In Manatara City, throw tomatoes at Askel.
  
• In Giant's Hall, beat up Bagogue.
  
• During the "A Spy in Mantara" side-quest, accuse the counselors.

Job Well Done!
Honor a Contract.
Earned for finishing a single bounty.


Compromising drawings
Help the Sister of Secrets.
This is a secret quest that is unlocked for going through the Rosalya doorway in the Arachna Coliseum.

Bounty Hunter
Finish all secondary quests and all contracts.
You have to finish the following 18 quests, and 8 bounties for this achievement to unlock:

• *A healthy stroll (Meonian Peninsula, Mantarian Camp)*
• He's running, he's running... (The Forest of Meonis, Cunning Caves)
  
• Catch 'em all (Gendys Plain, Shuffle Crossroads)
  
• ***Investing in the Past (Gendys Plain, Pouis Forest)***
• ***Bolts of Lightning in Shjue (Gendys Plain, Shjue)***
• Marys marries a mug! (Gendys Plain, Shjue)
  
• Music Maestra! (Gendys Plain, Shjue)
  
• The anger of Kaan (Gendys Plain, Shjue)
  
• Every rose has its thorns (Gendys Plain, Shjue)
  
• Who's your daddy? (Gendys Plain, Pouis Forest)
  
• Defend the Pouis (Gendys Plain, Pouis Forest)
  
• Eat me! (Gendys Plain, Shuffle Crossroads)
  
• ***Isolito apple thieves (Gendys Plain, Rewald's Place)***
• Isolito Reprisals (Gendys Plain, Rewald's Place)
  
• The Isolito Problem (Gendys Plain, Rewald's Place)
  
• Gambari Hunt (Gendys Plain, Rewald's Place)
  
• **A spy on Mantara (Mantara City, Lower Spaceport)**
• Sisters of Secrets

• *Koclibabs (Mantarian Camp, Lv6)*
• The Other (Mantara City, Lv20)
  
• Gittbris (Manata City, Lv24)
  
• Diamontos (Mantara City, Lv28)
  
• Gambarasaurus (Mantara City, Lv31)
  
• Laukhy (Bloodor Field, Lv34)
  
• The Devourer of Dreams (Gendys Plain, Lv42)
  
• Karok - The Third Eye (Mantara City, Lv47)

*Must be done before fighting Meos.
**Must be done before fighting Mother.
***Has two endings or leads to 2 different quests***

Major class
Kill an enemy with the Hyper attack.
You will unlock Hyper attacks after you go through the Cunning Caves, press R1 to activate them when you have enough stamina.


Raw Talent
Finish a fight without getting hit.


A class apart
Win 50 fights without getting hit by the enemy.


Beat the music
Defeat an enemy using Engagement only.
When you've progressed through the story, you can go back to the starting area, and kill an enemy in 1 hit while engaging them.


You can’t stop me!
Inflict 100 guard breaks.


Old school style
Win 100 combats without using magic.


Shi means to live!
Win 100 combats only using magic.
You can go to the starting area, and lock on to a creature and then cast a spell to initiate a battle, if the creature doesn't die from the first hit, cast another spell until he dies.


Some like it fat
Beat a legend with rank S.
You can do this on any of the Story bosses (Meos, Mother, Irys or Kaorys), I recommend Mother because she is the easiest and fastest battle of them all, just try to hurry through her battle sequence, and try and take as little damage as you can.


S For Sensational
Achieve rank S 100 times.
If you've progressed a bit through the story, you can go to the beginning of the game, and 1 hit every creature, which will result in an S rank.


Lazybutt
Achieve rank F 10 times.
Get your ass kicked a lot, and you'll earn this right away.

Elementary, my dear Poky
Discover 10 Interest Points.
There are probably around 20-30 Interest Points scattered through the game, I don't think this will be an issue.


The Explorer
Solve the puzzle in the Room of 9 Pillars.
There are 6 doors throughout the game, each with a symbol of one element. You can open a door if your affinity for the respective element is high enough, you can check your affinity in the Team menu of the game, a door requires 50~ to open.



Another world
Discover all of Rosalya’s doors.

• Gendys Plain, outside Shjue (Story related)[i.imgur.com]
  
• *The 3 Arrows, back of train across two acid pits*[i.imgur.com]
  
• Meonian Peninsula, across from Old Windmill[i.imgur.com]
  
• Shiyawo Mountain, just before leaving the Palace[i.imgur.com]
  
• Forest of Meonis, Arachna Coliseum[i.imgur.com]
  
• Forest of Meonis, Tree of Concerns[i.imgur.com]
  
• Laotan-Piki-Lanku, behind the waterfall[i.imgur.com]
  
• Bloodor Field, Giant's Hall[i.imgur.com]
  
• Meos' Palace, on the floating rocks[i.imgur.com]
  
• Gendys Station, inside a broken box on fire[i.imgur.com]

*3 Arrows location is missable, be sure not to miss it.


Catch 'em all!
Catch one of every wildlife.

• Poui (bird)
  
• Croyaha (clam)
  
• Squirropig (squirrel)
  
• Hoppoyon (butterfly)
  
• Hedgon (hedgehog)
  
• Meona (spider)
  
• Bovamoth (buffalo)
  
• Serkan (slug)

Zoologist
Capture 50 wildlife.

Invincible
Finish the game (with NO Game Over).
This sounds worse than it is, there are only a few spots which you may die in. If two of your characters die, press escape and quit to the menu, and try again. Never fight with the 3rd one unless you want to risk it.


Serkan Gear Solid
Reach Adorya station without setting off a single alarm.
This achievement is missable, and you cannot be seen by an alarm while in the 3 Arrows, if you're spotted, quit to the main menu and load the game again.


Cow-Boy
Acquire an Amos.
You can get an Amos from the farmer in Shjue.


The Rumble King
Beat one of each kind of enemy.

• Plant
  
• Troll
  
• Tree
  
• Ghost
  
• Fish
  
• Soldier
  
• Skeleton
  
• Golem
  
• Child

Hard-as-nails
Drop down to 1 HP.
Easiest way to do this is to step in to traps until your hp reaches 1 (You cannot die from traps), and then initiate a fight.


Botanist
Have Kaorys decrypt a Plant Shi.
You can find Kaorys in the Melody Garden or Lightning Tower in Mantar City depending on your progression in the story, if you progress too far she will be replaced by another person, which still unlocks the achievement.


Eclectic Electric
Have Irys decrypt a Lightning Shi.
You can find Irys in the Lightning Tower in Mantara City, if you progress too far she will be replaced by another person which still unlocks the achievement.


Shi Master
Unlock all Spells.
The only hidden spells are inside the 6 elemental doors, see The Explorer achievement.


It’s really going down!
Collect 100,000 Uzu.


Square One
Spend 100,000 Uzu.


Traps for Dummies
Trigger 50 traps.
Just keep running into traps, they cannot kill you, and they will re-arm themselves in 20 seconds.


Picturesque
Create 10 Recipes.
If you speak to a merchant and select the 'Barter' tab, you can offer him ingredients for items and spells.


In the end
Reach level 50 with every character.
This will require a small amount of grinding after you beat the game, I won while level 48 on my first playthrough. You should get this while doing the rest of the side-quests especially the Sisters of Secrets.


The Ultimate Predator
Acquire the Hunting Shoes.
The merchant in the Sisters of Secrets cave sells the shoes.


Featherweight
Get knocked out 50 times.
Have a character in your team die 50 times.