I checked game again just for you but I definitely don't remember how to play or even how to reach the boss and I'm definitely not making a video guide.
Qu had 999 stats everywhere, I farmed the best food for a while. And Oz had a rod that has a chance to inflict 3 different kinds of statuses and a poison brooch that inflicts 2. Hopefully that helps. I do remember using some weird gear like that on him.
There's definitely *some* rng involved but I don't remember struggling for too long.

I am kind of skeptic about your experience, then. I just got to lvl 14 by using some save edit to dupe dancer hearts and try to get better odds at surviving. The fight just becomes unberably hard and there does not seem to be a consistent way to put silence on the boss every time. Would you mind fighting the lvl 20 boss again, beating it and posting a video? Also, showing your stats and gear.

Sadly it's been almost a full year since I last played this game so I don't remember all the game mechanics. I just remember the general tactic I used (poison for dmg and silence + debuffs to survive) since it was fun to figure out by myself. There's no guides out there after all.
It gets harder and harder but I don't remember a lot of rng being involved in the fight so I guess if you tweak your stats and game plan with trial and error, you'll find a way.

Ooooh, I did notice that silence could work, but I did not notice poison would as well. Silence seems to wark either half or 75% of the time. Do you need capped luck to make it 100%

Mostly poison and silence + any other status effects you can stack while focusing on survival.

I know this is a super later reply, but could you tell me how the hell does one go about beating EX-Spriggan from lvl 8 ownards? Best strat I have noticed so far is to use Jelly Mirror against Succubus, Carbuncle and Wraith, and I guess Guard Aura against Minotaur. Queen Bee and Slime just wreck my Qu, and her defense is almost capped.

I really liked all the endings, Doubles Core are super talented. I recommend you try out all their games on Steam.
Good luck if you plan on attempting the extra dungeon, had tons of fun with it.

Eyup... It's as I thought... The Qu route ending ends on a MUCH MORE HAPPIER NOTE than the Toris ending, thank goodness I prefer the Qu route best and do it after taking care of the downer Toris ending, the OzXQu marriage bit NEVER FAILS to make me happy, especially when Qu brings up when they eventually will have beautiful children together in the future...

Ah, I see. I had no idea since I feel guilty going after Toris after spending so much time with Qu, especially with my immeasurable love of slime girls, not to mention it feels like betraying Qu after what she does to save poor Oz in the Qu route...

Is the bonus route really worthwhile though? I honestly dunno what it entails since i've never gotten it since i'm not one for romancing Toris after all, just Qu due to my aforementioned unending love of slime girls after all.

EDIT: From what I see from e-hentai's gallery of this title's former chinese name, it seems that it is worthwhile, though i'll go with Toris route first, as I completed it on Qu's route before a long while ago, and doing Qu's route afterward with sooth my inner-feeling of betrayal that i'll feel toward Qu when doing Toris' route first. If only there was a way to unlock the bonus route without completing Toris' route, ah well.

It's an extra dungeon you unlock after completing all 3 routes, you access it from a portal next to your room.