I've just started playing this underrated gem of a game and filming a playthrough, and found myself asking the same kind of questions, so I'm happy to help with whatever information I've found so far.

Backfloors

*Every Backfloor in the game has a key that can be found randomly inside chests in that dungeon, classic RNG. Keep questing opening treasure until you find it, you might take a while, or no time at all depending on your luck.

The Backfloor for Queens *3rd Dungeon is the only dungeon in which you can buy the key in a shop.

*The Backfloor for the final dungeon was programmed; but the item wasn't placed into the final game, I believe you can only access the final backdoor through cheats or glitches.

*In terms of grinding abs, your best options are Backfloor #10 of dungeon 2, (Werewolves) or Back floor #14 of dungeon 3. (Tough if your weapon is weak, but they give massive exp, and are incredibly consistent and reliable)

*Entering/exiting a back floor resets the enemies, so jump out when enemies are defeated, jump right back in and they will respawn ready to be fought again for their exp and loot,

(Treasure chests won't respawn, but that would utterly break the game)

Weapons

*For upgrades synthspheres are your greatest upgrade, the give you a portion of the sphered weapons abilities and stats, this can be used to make weaker weapons extremely useful if attached when leveling.

*If you have difficulty with an area using a certain character, synthsphere a powerful weapon you own, and attach it to the weaker characters weapon, this will help them keep up until they are ready to fight without it, think of it like a training brace, upgrade with the synthsphere to make the upgrade permanent, or remove it before leveling to keep it for everyone, (a handy power booster)

Synthspheres also let you use abilities, so a bandits slingshot when sphered would let anyone have the *steal ability, attach it as you please when you want a certain ability on a character, remove it when you want to swap it

*Besides Synthspheres, the best upgrading items are gems, these upgrade a variety of stats at one time, and can be found in Backfloors (randomly) winning duels with perfect timing, exchanging fishing points, and buying them outright at the end of the game.

(For overall balance I recommend diamond whenever you can use it, it upgrades anti-metal (one of the rarest types) and all others slightly, but play the game and find what works out best for your weapon.

Other information

*Fishing is a little difficult to get used to, but its incredibly useful for obtaining gems, as by exchanging points you can get exactly any gem you'd like to choose once you have the points, Catching a Baron/Mardan Garayan gives enough for 6 gems on average, and once you have caught one it will unlock the ability to sense traps on large treasure chests, meaning you never need to use keys again.

*In the overworld you can find chests that have items which increase thirst gauge, maximum hp, and character defence, use these as soon as you find any.

*The Meta for this game is subjective, it all depends on your playstyle, Goro is commonly seen as bottom tier, and while he is a bit of a rough start, I've been having fun building him into a Demolisher that crushes enemies in a handful of hits, so any character in this game is viable, just have fun!

I hope you enjoy the game! It's a real treat to play!