I tried day 181 for laughs. Yeah, there's no way you're getting through this: there are
over 35 enemies (more than the amount of squares you could ever fill, even with copies), mostly 3*, and so beefed up that they almost 1HK everyone. Got the expected amount of scrolls and flames so calculations seem to be right. (Giving up is required at this point since the game is so confused you don't even get a loss when all pieces get destroyed.)
Having a whole reserve of Julias did not help one bit since they couldn't even spam shield skills quickly enough. Only way to win is by effectively making yourself invulnerable, but that will also crash the game at this point since you can't win either - just give up.
There is no winning team really - just remain chaste until the end (refuse to indulge
every single time you get the prompt - the first time will permanently disable this route), don't give in to lust, don't let hope/confidence hit 0 and don't have the walls break. Getting to day 40 on story mode is fairly easy, upgrade your units to 3* and get race/class bonuses for extra survivability/damage (races get a bonus at 3 and 6 units of the same race, classes at 2 and 5 - they can overlap, though it's obviously impossible to get all major bonuses with 8 pieces - having all 24 units deployed, which normally isn't possible, will allow for this to happen as there's just about enough classes and races available for all major bonuses).
Is there a way to level up your units? All my 1 star units are getting instagibbed.
What danielwain said. It's actually not very intuitive, but buy multiple instances of the unit and they will fuse. 3 1* -> 1 2*, 3 2* -> 3*, so 9 copies total (will take up some leftover slots for the 1* and 2*s, use the reserve and try to focus on a certain unit).
Multiple copies on the field don't contribute to the class/race bonus as previously mentioned (and non-active units in the reserve do not count either) but if there's an unit you particularly like, there's nothing preventing you from getting 8 copies of it, it's just slightly less effective. Though having 8 3* Annos and a whole reserve of Julias is fun, expensive, and very very luck-based to not have to roll over and over again.
(She also doesn't come over to Celine 8 times for her sexual frustrations, only once. Could have been really fun heh.)
Edit: Some units still are 1* and 2* at extremely late stages, but they're so overpowered they destroy everything in their path. Even a 3* almost immediately falls to a 1* at this point. Did not try Calvins, but I'm almost sure you'd need an insane amount of lust to actually change something and on day 181 you'd have at most 1000 max lust with the Lust Control passive and constant use of Endurance Training. Not even RNG can help here but if you have an insane strategy with regular units, please share, it would be hilarious to get this far - though this will also lag
really hard.
Edit2: You don't really need to farm a lot of scrolls but if you want to, you can simply give up after a few days (at least 6 so you get 2 scrolls and 15 flames) then rinse and repeat. Purchase the skills you didn't get when rolling with the extra flames from duplicates.