Time for the final part, which is mostly a boss rush.
Went with commander mode since it fits well with my team. Everyone (except Muse) has high LP so they can get knocked to 0HP many times and not die permanently. My physical attackers sparked a powerful formation technique in which they all attack together to deal big damage.
You're forced to kick out one of your party members to make room for some kid who joins temporarily even though you can't use him in the final battle, so the old lady is gone. Now for the final final dungeon.

>>537001915
I think the best way to use Muse in a normal first playthrough is to give her one-handed swords because one of the strongest techniques does not depends on the user's strength (and she can learn it herself). In commander mode she only uses magic, though.
I'm not doing a super fast playthrough this time but it's still only ~6 hours since I skipped some quests and know where to go, so training Muse would be too much effort. Not only do you need to grind up her weapon levels, her SP or MP will lag behind and she'll be out of ammo after a couple attacks. I also didn't bother learning the strongest techniques for the other characters, since I'd have to spam attacks until a character gets a flash of inspiration.