The Master Synthesist is mentioned several times throughout the game but it is actually fairly difficult to track him down without the assistance of a walkthrough. In order to access the Synthesis Shop you need to defeat an optional boss which can be found in Memoria which can only be reached on Disc 4.
Travel to the second colored-orb that allows you to Save/Tent (shown in the screenshot below). Prepare the team by equipping any equipment that absorbs or resists Water-based magical attacks. Equipping a Ribbon accessory works wonderfully here while a Protect Ring suffices to greatly reduce the amount of damage done. Make sure to equip each of your party members with the Clear Headed, Antibody and Body Temp Abilities (if they have them).
Travel one more screen into the ocean area and search behind the rock on the right side of the screen. There is no field icon that appears, so just keep pressing the X Button until a “Mysterious Voice” asks “Who dares disturb my sleep? Leave at once, or you will die.” Choose “Don’t Leave” to initiate the battle.
Boss Battle: Hades
Preparation is the key to this fight. Hades will hit the party with a bombardment of negative status effects that will absolutely destroy you if you are not prepared. Hades’ moves include:
- “Judgement Sword” which will automatically drop a party member down to 1 HP
- “Cleave” which does physical damage to all party members
- “Freeze” which inflicts the Freeze status effect, causing that party member to by instantly killed if struck with a physical attack
- “Mustard Bomb” which inflicts the Heat status effect, causing the party member to instantly KO themselves when they attempt any action (their turn should be skipped until the status can be removed)
- “Reflect” which casts a reflective magical shield on itself (which can be removed with using Dispel)
- “Powering Sword” which uses Curse/Doomsday after 3 turns
- “Doomsday” which deals Shadow-elemental damage to all party members
- “Curse” which deals physical damage and has the chance to inflict the Slow, Darkness, Confusion, Poison and Mini status effects
Hades is susceptible to the Slow status effect and having the Clear Headed, Antibody and Body Temp abilities will help to greatly reduce the amount of damage done to your party members. Try to Steal all four items from Hades before the battle concludes.
Hades will offer you access to his Synthesis Shop after the battle and will suggest that his best item that he can create requires to Pumice Pieces (which you should already have if you have been following this walkthrough). Be cautious though as you will require the Pumice Pieces in order to defeat the most difficult optional boss. The Pumice (which can be Synthesized from two Pumice Pieces) can be used to teach Dagger the most powerful Eidolon, Ark. It’s up to you how you wish to proceed.
Hades can also Synthesize the following:
Make sure that you purchase the Tin Armor if you obtained the Hammer by completing the Stellazzio side quest in Treno. The Tin Armor is the most powerful armor in the game stat-wise by a significant margin. The other items are entirely optional.