I noticed the config.exe myself, but didn't dick around with it except for the slider since it wasn't translated. (Slider is supposed to be music volume but doesn't work, which is aggravating because the game's music is really quite loud.)
For the floor 18 boss, only tip I 'll offer is blow a lot of damage on him very early. He gets much less dangerous after you deal the first chunk of damage, and Tenshi can start tanking stuff then. By the time my team was leveled enough to not pop 3 characters every time he used Heavenly Demise, Tenshi was pretty much invincible to anything but Rasetsu fist for the entire fight. Worry about buffing everyone after that move is off the table.
As far as the query about the game's complexity, I'll just throw this out there for anyone else reading: The characters are very well designed and not nearly as interchangable as something like say, MMORPG classes. For tanks for example at this point I have access to:
Tank 1: High HP, high defense, weak mind (Magic resistance), weak attack power but with one attack that pierces defenses well, and a strong self healing ability, and a very inexpensive targetted heal that removes status ailments and heals few hp.
Tank 2: High HP, defense, attack, mind and speed. Only two moves: A single target phsyical attack with no particularly special properties, and a self buffing spell that raises attack, defense and mind values but can cause paralysis and poisoning to self as well.
Tank 3: Incredibly high HP, neglible defenses, high attack but low speed. Several moves that have a chance of causing instant death with various elements, targeting schemes and levels of power, one of which also causes a slew of status effects, lowering enemy speed, defense, mind, and can cause paralysis and poison as well as instant death.
Tank 4: Low HP, incredibly high defense and mind, low attack and mediocre speed. 2 Averagish physical nature element attacks, an expensive single target spell with the unique ability to remove enemy buffs, and a self targetted buff that massively increases defense and mind, but can also lower speed massively, cause paralysis and silence.
Now, based on those 4 alone, there's obvious interaction between the healer and the two self buffers with drawbacks. Aside from that, equipment options include items to increase resistance against any status ailment or ability debuffs, which mitigate these effects as well. In addition to THAT, paralysis and slowing isn't necesarily even bad for these characters, since buffs degrade by 20% of their current level every turn that character takes, so a slowed paralysed tank with +100% def and mind is pretty much invincible for a very long time. On top of that, some of that characters have interesting dilemmas on where to place stat points. Tanks 2 and 3 can serve as effective damage dealers if you build their attack power, but tank 3 could also benefit from speed in order to apply debuffs, or HP to tank better. Tank 2 can benefit from attack, defense, speed, hp, almost anything. You could specialize in one area (Making her a physical wall for example, or a very faster tank specialized in switching other characters ina dn out) or use moderation. Tank one's healing abilities are based off of her attack power, so raising that might be worthwhile if you want to usse her as a tanky healer, or you could make her more tanky, or faster for more reactive ailment curing. That was just 4 tanks, out of a roster of 40 characters. 4 of the most similar characters you could find are that different.
Aside from all that, all 4 obviously have specific advantages and disadvantages both versus specific enemies and paired with specific characters. There are other characters with buffs, that suffer from ailments, that cure ailments, that heal, that have incredibly high or low speed, that can cause debuffs or ailments with varying levels or power against various stats. Frankly if the game's stats were scaled for player versus player, it would be one of the most interesting competitive games I've ever seen. As it stands, it easily has the best blobber combat and character building I've ever seen.