Alright, so, played this. Went on normal for most of the first run, before I got bored and switched to easy. The game isn't particularly hard... it's just tedious with how you have to manage a bunch of stuff that's made more annoying by the way stupid debuffs are all over the place.
It takes several play-throughs to unlock all the scenes, because you can only unlock 1 scene per cleared combat stage. Note that this involves using the Play-Again with harder difficulty thing. Which just makes things more tedious and slow, so Easy mode is genuinely recommended.
The gangbang ending is unlocked when you do all of the last level h-scenes in a single playthrough. Yes. All four of them. You can't do that if you do individual Playthroughs from zero and just go down a column until you hit one of the four, because you won't be able to unlock all four.
The gangbang scene is designed to only be unlockable via the New Game+ mechanic.
All in all, that ending feels kind of weird. Because...
The whole build up to that mess is all about Sayaka liking sex with Haru more and more with each passing day. While she's clearly becoming a complete and total sex addict, there's no particular indication that Haru can't satisfy her... and he's the one that brings her there in the first place. There's no build up towards trying sex with more guys or anything. It's just bam, suddenly she's a super slut fucking dozens of people every day. Now... I personally like super-slut protagonists, and I don't mind the whole, sex-for-power mechanic... but in this case, it's just bizarre. It's like the game was supposed to have sex options for more power with strangers that led into that ending, but it was rushed out without them, while keeping just the ending.
So yeah. To conclude... the game is kind of eh? Not a terrible time waster, but pretty far from amazing too.
Can't even be assed to drop a review, not worth dealing with that shitty little interface for this.