Okay, after doing a little bit of testing, I'm like 99% sure it's impossible to get under 30 seconds on Shironeko's trial. Unless you go through the route flawlessly, running the entire time and never bumping into a wall, not getting any encounters AT ALL, you're never going to get it. On the upside though, I'm pretty sure you only need to get below 40 seconds for the reward, which is easy enough.
Personally, I haven't found any trouble with them. Though, whether that's because I'm using easy mode or because I got the best(?) empty gems in the game from the casino the first time I could go in as well as two persona masks, I'm unsure of.lol we're on the same part of the game. I noticed the game got noticable easier after I grinded to beat the snow giant. Probably because it gives you 4 front girls and 4 backup girls. Lots of moves that hit all enemies or multihit. I probably don't need to cheat now when I can just atk up and then multihit.
Edit: I just got to the next boss and I retract what I said earlier. Maybe it's just the bosses that are overtuned. Seems to be immune to physical attacks which is a major pain considering that rules out like half your girls.