So to get harem ending I have to have both Nanako and Chiru go to heaven and hell for the ending or just doki-doki both only?

The order for the Harem ending doesn't seem to matter as long as you complete Risu's quest line correctly and then begin Nanako/Chiru missions. I finished Chiru's tasks first, then Nanako's.
You also seem to be able to switch quest lines as you go, but if you want to be safe in NG+, simply follow the guide's instructions and go home and skip the day if Nanako has nothing to say or isn't there. Nanako's quest line lasts around 7-9 days, as she often takes a break from school after her identity is revealed.

I also found out one easy way to check if you've cleared the quest line for Chiru or Nanako and cleared the requirement for Kurona's Return mission is to see whether Chiru or Nanako has the permanent exclamation mark ! in their places after they leave for heaven/hell .
If you don't see the dialogue saying things like missing them, then the quest line is not over yet so you might want to go to the next day or just play some free missions.

Does it matter the order?

@quicksolve83 you must doki doki risu and no one else yet in order to unlock kurona's return mission. Please read the walkthrough carefully.

Why do you never said about chiru/nanako boss battle? Do i need to avoid it? Also, how could i get kurona return mission? I've already done all main mission and chiru/nanako side mission but no kurona return, is that because i finished nanako/chiru boss battle or is it because i done their doki doki first before risu's?

Also, one thing that I've found only on Playstation guide for Gal*Gun 2 is that you need to do both ending A and ending B to get the girl's phone number. So if you are aiming for the walking phonebook achievement you have to do all endings, except the normal and harem one, in a single play-through (either as a series of continues or New Game+). If you save scum you won't have all phone numbers, if you go for regular New Game, that will delete all previous phone numbers you collected (including Risu, Chiro, Kurona, and Nanako).

Hey everyone the guide doesn't mention but if you do Chiru's route first before Nanako while going for harem the ending WILL NOT unlock. This doesn't seem to be mentioned in any guide but I just wasted 5 hours because of that

win10win11
collapse Is there any way to solve this problem

Thanks a ton for your guide, it was accurate and simple to follow but I'd change something there : in some cases you don't actually need to play side missions until you go home, you can go there sooner and trigger events instead, it was faster ( specially regarding nanako who makes you spend the day to refresh some of her events )

Kurona's return is a third and final encounter with Kurona that happens after you doki doki one of the two girls. She is ACTUALLY PRETTY HARD TO BEAT because it's a very fast paced fight.