Advanced Item Creation in Star Ocean First Departure

Firstly, remember to get Determination as high as you can, and level Purity if you need seed money. When you're leveling Specialties to use them, always stop at Level 8, going higher is a waste for a reason I will explain). Here's a list of all talents your characters can potentially learn by ease and the Specialties you need to use to unlock them (only need Level 1 to learn Talents, remember).
DO NOT INCREASE MUSICAL SKILLS PAST LEVEL 1 UNTIL YOU REACH THE STEPS BELOW.

Save before doing a crafting batch, so you don't lose your items if you get terrible luck

Roddick -
Love of Animals (Familiar - Whistling, Animal Training)
Rhythm Sense (Music - Performance and Music Knowledge [Learning seems to happen by Peforming, not Composing])
Taste (Cooking - Knife, Recipe, Keen Eye)
Nimble Fingers (Crafting - Mineralogy, Eye for Detail, Aesthetics)
Composition (Writing - Penmanship)
Design Sense (Art - Sketching, Aesthetics)

Ilia - Love of Animals, Rhythm Sense, Listening (same as Rhythm Sense), Composition, Nimble Fingers, Design Sense, Taste
Cyuss - Love of Animals, Rhythm Sense, Listening, Taste, Originality (same as Nimble Fingers), Design Sense, Composition
Ioshua - Rhythm Sense, Listening, Composition, Design Sense, Nimble Fingers, Taste
Millie - Rhythm Sense, Composition, Nimble Fingers, Originality, Design Sense
Ronyx - Rhythm Sense, Listening, Composition, Design Sense, Nimble Fingers, Originality
Mavelle - Love of Animals, Rhythm Sense, Listening, Nimble Fingers, Composition, Design Sense, Taste, Originality

With SP and money in hand, do this:
Buy a Violin from the music shop (Eckdart), and a Lezard Flask from the craft shop (Ionis)

Step 1: Music
Level Muscial skill on someone with Listening and Rhythm Sense Talents up to 8. (I recommend Ioshua, as he has nothing else relevant to contribute) Buy a Violin, and compose from the Specialty Menu with him (need Feather Pens from music shop) until you learn the song "Goddess' Arival", which boosts the success rate of all Item Creation while it is playing.

Step 2: Alchemy
Give Ronyx Alchemy Level 8. Have Ioshua play Goddess' Arivial (need Composer's Batons from music shop), and then have Ronyx attempt to make Rainbow Diamonds via Alchemy under Item Creation (need Iron from the craft shop). You need four Rainbow Diamonds total.

Have Ronyx and Ioshua both wear two Rainbow Diamonds as accessories. Ioshua will keep his forever. This temporarily raises all their Specialty levels by 2 (so they have 10 Music/Alchemy). Play Goddess' Arival again and make as many Philosopher Stones as you can. Sell them for infinte Fol. Then, make 1 Damascus for Roddick's ultimate weapon later (can't make it yet.)

Step 3: Crafting
Have Roddick or Illia learn Crafting Level 8, then give them Ronyx's Rainbow Diamonds (to reach Level 10) and have them Craft Rubies (buyable at the Jewel Shop in Eckdart) while Ioshua plays Goddess' Arival until you get 3 Berserk Rings (one for Roddick, Ilia, and Cyuss). Do the same with Green Beryls (also buyable in the Jewel Shop) to get 4 Faerie Rings (one for everyone).

Step 4: Customization
Customization can be used to turn a weapon and a jewel into a vastly stronger weapon. It comes from the Skills Eye for Detail, Smithing, and Aesthetic Design, and uses Originality as a talent, otherwise success chance takes a big hit. Each of your four characters must customize their own weapons.

Once a character is at Level 8 (using the spare Rainbow Diamonds to reach 10), have them spam combining whatever cheap weapons you can buy with stuff from the Jewel Shop (Diamond is the best) while playing Goddess' Arrival until you get something you like. For Roddick, Sword of Marvels or Souleater are the best outcomes by far, for Millie, Rod of the Feather. Cyuss has all sorts of crazy elemental weapons, and though Illia will struggle without Originality, the music should give her a decent chance to get solid gear.

Step 5: Extreme Leveling
Now that you are geared up, you can gain levels at an absurd rate.
Buy a Piano and a Silver Trumpet from the music shop and have Ioshua compose the Devil's Aria on the Trumpet and The Celebration Hymm on the Piano. The Devil's Aria will summon allegedly powerful monsters when played on the World Map that give insane amounts of EXP. The monster you summon is based on the musical level of the player (the monster summoned at Level 10, is the hardest boss, so don't have Ioshua play it unless you want to die horribly).

Get anyone else with both Rhythm Sense and Listening to exactly Level 4 Music, save, play the Devil's Aria with them, then have Ioshua play the Celebration Hymm before you exit the menu. With your powerful gear (assuming you have at least one fighter with a non elemental weapon, as the monster absorbs all elements) and the musical boost, you should kill it with relative ease and skyrocket in levels.
Repeat to your heart's content.

Step 6: Reducing the Encounter Rate
If you want to reduce the encounter rate, have someone (probably Cyuss) with the Animal Instinct talent learn Scouting from the skill Danger Sense and then use the Specialty to cut it. To reduce it even more, you can Craft a Philosopher's Stone into a Ring of Avoidance with Mavelle, and a Crystal (Jewel Shop buyable) into an Intimidation Pendant with Ronyx. Put one of each on someone in your inactive party, and along with Scouting you will have the minimum encounter rate.