Good to see that you also consider Koishi as an iji boss. I still prefer Suwacko over the other two though. Her battle is overall more fast-paced and I like Native Faith better.
Zengeku and you are worlds apart from me:
SA Extra:
Hundreds of tries and no victory. However, I love the stage a lot and I absolutely love Koishi's character design as well so that alone makes me want to keep playing.
Did I mention I also love her stage theme. Spell cards are among the prettiest I have ever seen in Touhou. Flower Danmaku <3
MoF Extra: Tried a few times and put it away. It's not difficult. I die because I don't know the cards. No thanks, I really don't want to go through another memorize-fest just to beat this stage. I don't like Suwako at all either. Kanako is so much cooler.
I would have said the same thing about Mokou, but IN has Spell Practice, making beating the stage much faster.
PCB Extra: 12 tries and Ran is down. PCB Phantasm: 7 tries, get to Yukari's last card, but I kind of put it away for a bit.
UFO Extra: Not even going to start on this. I love it, though.
Koishi -> One of the hardest Extra. I can capture most of her cards, but that's due to having triple-digit history.