This will be a hard boss "fight", thus you'll need to prepare accordingly. If you need to, you can spend an infinite amount of days buying necessary ingredients and tools before you actually attempt the final boss event.

Before you start:
- check that you have ingredients to make at least 50 of about 3 expensive food items, possibly made with common ingredients that you can easily buy or find
- make sure you have many different ingredients in your inventory, even stuff you never used before, they might be useful
- if you used to run your izakaya always in the same place, try to run it in different places so you can gather info on rare guests. Any special or DLC guests shouldn't appear in the final boss event, so you can ignore them.
- buy "fairy maid selling leftovers"' full stock at the shrine (it has some useful ingredients)
- check if you have gold and night sparrow tools, they are immensely useful. They are sold at the Human Village.

When you are done with your preparations, run a lv3 izakaya anywhere and add a preset with your 3 "expensive food items, possibly made with common ingredients that you can easily buy or find", mine were:
- kabayaki lampreys
- tonkotsu ramen
- dumplings
Then add 4-5 expensive (from 40 to 100 yen) drink items that you have a lot of, and all the tools you'll use. Don't just add one each, make sure to add duplicates.

In the following day, cook 10 each of your 3 chosen food items, so that you can possibly serve all customers in phase 1 without cooking the food during the fight.
Now, onto the final boss!

You'll serve normal guests, each of them will ask for 1 food and drink from your lineup, and they will make multiple orders.

- Load your preset with only your 3 expensive food items for your menu, so that they are forced to ask for those, and the 4-5 expensive drinks, plus your tools
- During p1 cook some items for phase 2 (see Phase 2 paragraph for more details). Try to cook as many as you can while you serve this phase's food and drinks.

This is your nightmare. You must serve special guests, trying to get reward spell cards (by cooking them food with more than 2-3 tags they like and cointains one tag they asked for), and you need 5 to complete the round. It is crucial to know what you are doing, because you have little time. Every 10 seconds a punishment spell card belonging to a character currently at your izakaya will be activated. Some aren't so bad, while others can prevent you from cooking for a few seconds (wasting the little time you have) or scaring away the customer you were cooking for, or everyone apart from the spell card user. You should try to serve the most dangerous guests with food they at least partially like so they can leave as soon as possible. And yes, this is RNG. Good luck, you'll need it.

- keep the 3 starting items for p2 (a spell card can prevent you from cooking 3 items among the ones you add to the menu)
- while you cook what you need to serve, try to cook other items as well, the guest that wants them might appear at some point
- some guests will order 2-3 times, if you know what they love, you're set. Just make sure they aren't ordering something different each time (example, if a character wants a No Alchool drink, you can't give them a Low Alcohol one, even if it contains another tag they like)

Here's my personal list of combinations I have tested and will work:
wriggle: salmon steak - egg cicada meat + fruity highball
suika: boiled tofu - meat daikon + tengu tango
reimu: riceball - lunar herb premium tuna + scarlet tea or cassia wine
rumia: tonkotsu ramen - egg + mio
hieda no akyuu: dumplings - tuna + tengu tango
hieda no akyuu: rice ball - premium tuna - tofu + tengu tango
chen: pork & trout kebab - honey + mio
keine: tangyuang - meat + sangetsusei / divine wheat (? untested drink)

This was recommended here on Steam, but I haven't personally tested it:
Patchouli: Udumbara Cake / Unconscious Youkai Mousse + mio (? untested drink)

Here's my list of combinations that have not worked at all times, but that you might want to experiment with:
wriggle: tonkotsu ramen - cicada honey + fruity highball (might work with a night sparrow tool)

Here's a list of combinations guests like, but need more tags to work, maybe you can try and fix them:
mokou: hodgepodge + mio
[amii suggests: if you want to improve hodgepodge for mokou, try adding chilli and peach/grapes]
ice fairy: scarlet devil cake + fruity sour
chen: lamprey + scarlet tea
chen: kabayaki lampreys - honey - meat + mio
patchouli: dumplings + mio
hieda no akyuu: dumplings - tofu + tengu tango
hieda no akyuu: pink rice ball (remove hot tag and it might work) - tuna + sangetsusei
kaguya: dumplings + ramune
inaba: small portion trout + affogato

If you made it here, congrats! Now, you'll just need to serve Yuyuko a ton of food until she is satisfied. Apparently she needs food that require 5 ingredients to be made, possibly the event ones you received about a week before the final boss, so add those to the menu.

- make sure to add the 2 event 5-ingredent food plus at least 2 grills and 2 pots
- add or keep tonkotsu ramen (useful conbo I used: ramen - cicada honey)
- cook at least 4 food items all at once (using said pots and grills) and make sure you never run out of food items, and quickly serve them all to Yuyuko.
- if you only serve these and never waste time, you should be fine.
- as a drink, I served Godfather, but I'm not sure it makes a difference.

After this, you might get an optional boss, but as you can imagine, it's not needed to complete the game.
If you made it this far and completed the boss fight, congrats! Just follow the game's journal and it will tell you how to trigger the final cutscene.

I hope this guide helped you, if you have any suggestions I'll be glad to add them crediting you!
Blisscast