Do the following task to unlock the achievement and it's alloted gamerscore.
Unlockable - How to Unlock
All Barriers Released (25) - Release all barriers
1,000,000 Gold Accumulated (25) - Accumulate 1,000,000 gold
100 Back Attacks (30) - Successfully pull off 100 back attacks
100 Monster Fights (30) - Set up over 100 monster fights
All Characters Level MAX (50) - Get all characters to level 99
All Warp Clip Set (25) - Activate all warp monuments
Dragon, 1 Category Rank MAX (30) - Max out the rank of one category for Dragon
Dragon, All Category Rank MAX (50) - Max out the rank of all categories for Dragon
Item Dictionary Complete (25) - Complete all entries in the item dictionary
Mechat Shooting, ST1, Perfect (30) - Clear Mechat shooting with no damage
Minotaur, 1 Category Rank MAX (30) - Max out the rank of one category for Minotaur
Minotaur, All Category Rank MAX (50) - Max out the rank of all categories for Minotaur
Monster Dictionary Complete (25) - Complete all entries in the monster dictionary
Phoenix, 1 Category Rank MAX (30) - Max out the rank of one category for Phoenix
Phoenix, All Category Rank MAX (50) - Max out the rank of all categories for Phoenix
Rare Item "Dragon Bracelet" (10) - Find the rare item Dragon Bracelet
Rare Item "Ultimate Conflict Necklace" (10) - Find the rare item Ultimate Conflict Necklace
Rare Item "Ultimate Weapon Bracelet" (10) - Find the rare item Ultimate Weapon Bracelet
Rare Monster "Gold Mecha Robo" (30) - Defeat Gold Mecha Robo
Rare Monster "Golden Unchi-Snake" (30) - Defeat Golden Unchi-Snake
Rare Monster "Ousama Unchi-kun" (30) - Defeat Ousama Unchikun
Simultaneous Encounter, Over 10 Parties (30) - Successfully pull off over 10 parties of simultaneous encounters and defeat all the monsters
Successful Back Attack (5) - Successfully pull off a back attack
Successful Simultaneous Encounter (5) - Successfully pull off a simultaneous encounter
Wagon Defense, Perfect (30) - Clear wagon defense with no damage
Corporeal mode
After you have taken enough damage, and you yourself have inflicted damage, there should be an option above the attack that reads 'Corporeal'. It basically turns you into a real monster for a short while, and you will auto kill anything around.
Defeating Moody Dragon
You can find the hidden Boss, Moody Dragon, while leveling up your characters in the Underground River. This Boss is found at random, so this may take some time. Try being at level 27 or above to defeat this Boss. Just use physical attacks, or anything except Shadowa. He has about 2,500 HP. Upon defeating this boss, you will get a nice reward.
Don't Miss Out
If you are looking for the All Item Achievement, you will be looking for all the 1800 "nothings" and trading them to the Nothing Man in Jibral (or the Nothing Robo Mecha in Split Worlds) for items; exploring every other chest in the game will pretty much help augment your item collection. However, there are two accessories you must steal from two different bosses or you will not complete your item collection.
Be sure to steal from the Phantom Dragon south of the Glass Spires on disc 3 for one accessory.
Be sure to Loot or Steal from Ultimate Szabo in the Primitive Cube (Cauldron Room) on disc 3 for the second accessory.
Easy experience
Immediately after the scene where the strange voice tells your characters to swallow the light spheres (after fighting the land shark), there is a chance to gain at least three or four levels in both class rank and character level. You have the choice to escape using the Mechat machine, but do not. Instead, fight the horde of robot enemies. This may take awhile, but the experience gained can really help your characters for what lies ahead. The Underground River area on Disc 2 is a good location to level up your characters fairly easily. Go to the end of the area where you can find lots of the Kelelon things then just battle them again repeatedly. They will always respawn. Try to use R to surround as many monsters in your radius as possible. The Underground River is also well known for having lots of other respawning monsters.
Easy SP
When you first visit Nene's castle, you will come across red tripwires that, when tripped, will result in an enemy encounter. After you fight the boss in the castle, you will come across a long room with a treasure chest within. In the very beginning of the room, there is one of the aforementioned red tripwires which will result in an enemy encounter if tripped. If you use Field Barrier here, you can run back and forth all the way down the hall and back to the tripwire, you can spawn a new enemy encounter each time, which is an incredibly easy way to earn SP.
Go SP Yourself
The Barrier Magic has a field skill "Field Barrier 3" which only costs 1 MP (instead of 10) at rank 50. Once you earn and assign this skill, you can venture to places with plenty of enemies you've defeated before and simply walk into them with the Black Belt and Eyepatch accessories (the skills Attract Aura and Regenerate MP at this point are self-explanatory). Some good places would be:
Underground River - the Kelolon Defender Room, all the way to Baroy Town (80 enemies)
Primitive Cube - the Blazing Moths in the garden (infinite but slower number of appearances)
West Ice Field - the Roball racetrack (dependent on the moving base's appearance)
Invincibile Jiro
When you use Jiro in battle and he dies, he will automatically resurrect himself.
Items
Items are not only useful for heals and revives. The attack item is useful for killing enemies all at once. The support option is helpful when you need to help your allies or you need to make an enemy weaker or debuff them.
Rare Monsters 'R Us
Aside from the 20 to 30 monsters you can miss in Nene's Fortress (end of disc 2), there are three elusive monsters you will have to search for once your party gets a mechat on disc 3.
These monsters are super tough and can do more than 999 damage to everyone in your party. Increase your shadow class ranks or expect to die very quickly.
Gold Mecha Robo: The Ancient Factory 1F. Past the first conveyor belt.
King Poo Snake: In the Ancient Ruins Cave. Defeat the Jumbo Poo then the Corrosive Poo for this enemy to appear.
Gold Poo Snake: Defeat the King Poo Snake and the Platnum Poo. The Platinum Poo is on one of the three south and southwestern island archipalageo and the Gold Poo Snake will rarely appear (on that same set of islands), giving you that chance to destroy it.
Secret Achievements
Complete Mechat Shooting Stage 2 perfectly - 30 points
Destroy all machine guns on Mecha Base High and take no damage while completing Mechat Shooting Stage 2.
Complete Mechat Shooting Stage 3 perfectly - 30 points
Complete Mechat Shooting Stage 3 without taking any damage.
Get Surprise Attacked 50 times - 0 points
Allow monster parties to attack you from behind 50 times.
Install all Mechat upgrade parts - 10 points
Obtain all nine Mechat upgrade parts and then install them all at the Mechat Dock. You need not install them all at once.
Moon Laser Perfect - 30 points
Complete the Moon Laser mini-game and receive the "Perfect Bonus". (You cannot take damage or hit any of the moons for 0 damage.)
Obtain all dragon items - 10 points
Defeat all five dragons and open the chests they leave behind. This achievement unlocks when you obtain the fifth item.
Reach MAX rank for all of the Killer Bat's categories - 50 points
Reach rank 99 in all nine categories for the Killer Bat shadow. Do not use a heart item to go from rank 98 to 99 in any category.
Reach MAX rank for all of the Saber Tiger's categories - 50 points
Reach rank 99 in all nine categories for the Saber Tiger shadow. Do not use a heart item to go from rank 98 to 99 in any category.
Reach MAX rank in any category for the Killer Bat - 30 points
Reach rank 99 in any one category for the Killer Bat shadow. Do not use a heart item to go from rank 98 to 99.
Reach MAX rank in any category for the Saber Tiger - 30 points
Reach rank 99 in any one category for the Saber Tiger shadow. Do not use a heart item to go from rank 98 to 99.
Run away 50 times - 0 points
Run away from 50 different encounters.
Successfully arrive before the time limit expires - 5 points
Reach Nene's throne room at the top of Nene's Fortress before the time limit expires.
Successfully close the door - 5 points
Complete the "close the door" dexterity game in the Giant Mechat without allowing any Mecha Robos to get through.
Successfully escape from the Ostrich Mecha Robo - 5 points
Complete the Ostrich dexterity game in Nene's Fortress without allowing it to stomp on you and then select run.
Successfully make your shadow emerge - 5 points
Complete the shadow dexterity game in Devour Village in a short time. (Do it as fast as you can.)
Successfully pull in the Giant Mechat - 5 points
Complete the "pull in the Giant Mechat" dexterity game without angering any of the Gohgohs.
Successfully remove the neck ring - 5 points
Complete the neck ring dexterity game that occurs in Nene's Fortress in a short time. (Do it as fast as you can.)
Successfully save Kluke - 5 points
Press the indicated button one time each time it appears on screen during the event when Kluke is thrown off the giant green Mecha Robo.