Before anything else, raise that stamina ASAP. At the very least, you should have 6 Stamina, which is the maximum amount that gets recovered every day. Ideally, you'd have at least 10, which is that same 6 max per day, plus eating a single Pumpkin Pie (you can make one of those every other day).
After that, the actual costume is irrelevant, chose whichever you like the most (I wen't with the cat-eared Halloween costume). You need all the costumes just to unlock new techniques in the shop, but you don't need to actually dress up for them.
You're pretty much at the end of the current available content. The Halloween party and reaching that 15% Lust should be the last thing you have on hand.
As such, take some time to maximize everything available:
- Train Emily to max Corruption (Current max is 13, I believe you'll get her last missing costume at some corruption level)
- Get Emily to max Stamina (Current max is 14)
- Get all of Dante's armor pieces
- Get every single costume for every girl
That way, you'll be at max strength whenever we get the next update.
See the attached pictures for the full list of all upgradeable variables at max values
Edit: Reuploading attached images (first time I borked them)
