Last time, I wasn't just doing this event and nothing else. I was doing the other normal stuff to get stronger, with convoys and leveling lab skills such, and only doing the event (then called the crusader dungeon) once I had run out of other things to do. I got my Tiger of Kai to a high enough level to be useful during that event, which helped quite a bit.
If it takes you 9 battles to beat the end boss, and those battles are probably quite a few rounds each, then you're right to ignore the event and focus on the other one. If you've got a much stronger party by the time it runs again, then you can really take advantage of it. Even for clearing the boss in one battle, there's still in huge difference in the time it takes between 1 battle that takes 5 rounds and one battle that only takes 2 rounds.
My experience with this game is that you get stronger much faster than you expect. It seems like it's rarely more than a couple of weeks from "I can't beat opponent X at all" to "I reliably get an A grade or better against opponent X and am now stuck on something else instead." If you're taking 9 battles to beat the end boss, you've still got a ton of things you can do to get stronger. You might not even have a full team of level 55 generals yet, and surely have space to inherit generals, move from SR to SSR, get stronger gear, level up lab and church/temple skills, forge your gear to higher levels, and so forth.
All of those things slow down after a while, but even at the daimyo-contender level, just getting another inherited Takeda Shingen of Kai makes a huge difference. I don't see it really slowing down until we've got parties with an inherited SSR Takeda Shingen of Kai for each of the five generals on the team, or possibly only 3 or 4 if you think the Uesugi Kenshin/Takenaka Hanbei combination will be strong enough to justify forgoing one.