-( Story related, cannot be missed )-
The first dungeon, and your first real excursion into the Midnight Channel. Yukiko's Castle isn't too dangerous (unless you're just rushing through and not taking your time), but the mini-boss, and the boss itself can be threatening if you take them lightly. This trophy will unlock after you've successfully rescued Yukiko and brought her back out to Inaba.
The boss of the Castle is susceptible to Ice, but Chie (in her early levels) will burn through SP quite fast. If you also add in the fact that she is the first to employ a lackey in her boss battle (which you will see quite often later), the minion will be the more formidable of the two. Defeat him as fast as you can and he should give up the call later. Meanwhile, the boss is slightly predictable with some of her more deadly attacks, so make sure to defend when necessary.