Whether you know the "Expected Result" of a synthesis combination is stored in a global save, so it will be remembered even if you reload your save (and on repeat playthroughs). If you have limited resources, you can keep creating and reloading to fill out all the ???s and see what items you want to make most.

There are two chests at the start of the game which you can reach by making a Bomb (Item, Magic Stone + Shimmer Stone). One of the chests has a Transfer Crystal, which you can't easily make yourself for a while. It lets you warp to the start of the current area in an emergency.

The LP upgrades at Nobody's Home will take you up to 10 LP. If you're willing to grind out the souls, you can purchase up to max LP from the start of the game. The equipment slot upgrades are limited based on progress. At first, you can only buy up to 4 slots; after reaching Plate 3, up to 5 slots; and after reaching Plate 2, up to the max of 6.

In the room of eight torches on Plate 2, the top-left-most, top-right-most, and two bottom-right torches will open the doors. The other four will create permanent monster spawns.

There are a total of 20 captive humans in the game. Killing them or freeing them affects a sliding scale that determines the ending. Killing humans gets you souls, which increase in number the more you kill them. Freeing them offers no special rewards. After going 10 in either direction, you can no longer choose the opposite option. After 15, you can't even choose "do nothing."

The three main endings occur for being 15 or more on the "kill humans" side, 15 or more on the "free humans" side, or being somewhere in the middle. For minimal effort in seeing the endings for both extremes, you should probably just not choose to free or kill anyone, but go back and do so at the end.

The merchants at the end of the areas can also be killed, which is 3 steps toward the "free humans" side of the scale. Killing them gives you an item instead of souls, usually an Immortal Potion.

A difficult fight and secret ending lie beyond the bombable door in the back of where you start the game. You can go there anytime, but it's "guarded" by the same miniboss you fight before the final boss, and naturally you'll want to be as well-prepared as possible.

Corpse Eater

Like the name implies, she eats her allies and gains stats from them.
So take them out quickly to keep her from getting too strong. The Skin-Peeled are a little weak to Holy.

Blood Beast

He's weak to Holy, so Heavens Ray is recommended (from the Silver Bullet weapon).
He's also susceptible to Charm and semi-susceptible to Weaken.
Try to take out the summoned undead quickly.
When Blood Beast dies, he uses Too Soon For Relief!, which gives SP to any survivors.

Salvationer

He's susceptible to Weaken (and somewhat to Numb), so stack it on with Dies Irae.
If an Offering is still alive when he dies, it uses Goodbye, which wipes out your party.
Obviously, this means you must take out the Offerings first. (They're also susceptible to Weaken.)
He uses Wanderlast a lot to pile on many status effects, so try to have some protection.
He uses Good Night every 3 turns, which is basically a knock-out unless guarding.
Thus, it may help to keep summoning undead to reduce the odds of Jeannie or Marta fainting.

Queen Anastasia

Beauty Queen is slightly susceptible to Weaken.
Ash Queen is slightly susceptible to all status effects and Holy.
Immortal Queen is very susceptible to all status effects, especially Weaken.

Pleiades Knights

Well, good luck!

Synthesis basically works like this: There is a predefined list of all equipment.
Each entry has adjectives associated with it, including a minimum level and category (item/weapon/armor).

When you put together materials (each of which have their associated adjectives),
it searches through the list for a piece of equipment which satisfies the "input adjectives."
If there is no exact match that satisfies all the input, it falls back on the "best match,"
the equipment that has best satisfied the greatest number of input adjectives.

An equipment's "level" is the lowest level of stone that can be used to make it.
For example, Silver Bullet is Lv2, so while it's just a Stone and a Cross, you can't make it with a Lv1 Magic Stone.
There is no benefit to using a higher-level stone than necessary to make equipment;
the level of the stone does not "carry over" or grant any extra bonuses.

Level 1 Equipment (Magic Stone)

Items

Weapons

Armor

Level 2 Equipment (Spirit Stone)

Items

Weapons

Armor

Level 3 Equipment (Heroic Soulstone)

Items

Weapons

Armor

Level 4 Equipment (Divine Spirit Stone)

Weapons

Armor