Easiest way of getting her into trance is spanking. If spanking is unavailable (whether it's not unlocked yet, or just because pose didn't allow it) - go with the most intense options and just spam it, like hand massageur, as an example.
I usually always tried to keep some variety in actions, making rotations of tons of different actions, thinking that score will diminish if it think that what i'm doing is "too boring"/"too monotone", but as far as my testing goes - that's not the case. In fact, it seems the other way around - the more you concentrate on one area - easier it is get her into trance, and getting same action over and over again didn't diminish final score at all. The trick to the trance state is not allowing her a break in "intense action" that you are doing (ofcourse you still need to let her accustomed to it first, for it to not filling her uncomfortable gauge).
Also, rank is coming from average score rate, aka "score per second". The reason why short sessions tend to give less rank is because you need time to prepare her at the start, which is usually very low score endeavour, but average will went up after you get to actual intense action afterwards. So, the trick to S rank is to prepare her as fast as possible, and as deep as you can, and then spam the most intense techniques as much as possible. For that need to know what can be used at the same time.
After she get comfortable with spanking, trance state is absurdly easy, and just spamming anything will get you easy S rank. So that's my main advice - prioritize her "ass" exp as much as possible, until you get spanking and get her used to it.