Guys i just realised playing the game in insane difficulty is actually easier for you to take down the heroines without any damage done to you. Because all the other difficulties shares CD, so even if your CD is off sometimes the heroine cuts queue and land a few hits on you, making the zone control drop and chopping off your hp, plus some annoying debuffs. Playing on insane difficulty, however, your skill doesn't share cd with the heroine, meaning if you pack yourself with the acid rain (max 6s CD), which is also the best zone control with decent damage, and you pack like a bunch of them, I loaded 9 on each heroine. What you will see is rain of acids continuously non stop and the heroine get raped before she can let out any skills. Plus you will be sitting at 100% zone control. Neat stuff.
Anyway, i think i have found out most of the requirement for all the cgs, basically you will have to corrupt all the heroine at 100% without the key word at different stages of heart, and if you are sitting at high likability with the heroine ( clearing all the stars event + going out on dates on sunday) you will be able to unlock all the scenes for that particular heroine on one run. I suggest focusing on one heroine per time and do 5 runs to collect all the cgs. Or you can do it like me, save halfway and complete one route, then load the save file and change to the other route.