Pretty sure I was 130-140 with Reimu on the final boss. It DID take forever though, I said I'd spend over an hour fighting only to wipe at the very end, I wasn't exagerating.
The add on the right is indeed beefy as @%@#%@. Also in case you haven't noticed, the more you damage the boss (and her adds) the faster she gets (and I don't mean speed buffs simply). Also, for every add you kill, the boss, as well as her adds gain new abilities and power. This is completely gradual, so there is no point in trying buff up before a certain threshold or whatever.
From what I've seen:
top add: I like to kill this one first (it has very little health too), even though the one on the far right is heavily damaged by the time it comes out, this is because it spams status ailments, I really hate the toxin one. Even with over 30 poison resistance, it brings people with near-20k health to 1 in no time flat. If you decide to try killing this one last, it can cast djinn storm...which might not be too bad since the boss can cast it too.
Left add: Doesn't seem to do any new tricks as you kill more adds other than "magic jitsu" after you kill the other 2, which buffs mag by 100%. Even still, it doesn't seem very threatening unless it casts ether flare, in which case it hurts somewhat. But still is a reasonably powerful enemy. 100% mag from magic jitsu followed by ether flare does NOT result in you getting screwed, thank goodness. Note, the add is **RIDICULOUSLY** weak to fire. My royal flares would cream it for 200,000. It doesn't sound like much now, but at that time a full sp 100% mag master spark would only hit for 300kish for me. Speaking of master spark, I recommend using it on the top add as soon as it spawns if you decide to kill it first, otherwise use it on the right. Don't hoard it for the boss herself after, it will no doubt be wasted by a djinn storm.
Right add: ***DO NOT*** leave this one last intentionally. If you do, it will eventually spam scourge at some point...That move diamond knights have which its for 200,000. I notice later on in the fight it can cast something alot like sakuya's soul-sculpture. This move is physical and it hurts the entire party. While hyperspace slash and samidere slash might not waste physical weak casters in the back, this ability most likely will. I really suggest killing it asap after the top middle is down
After you kill all 3 adds, the boss gains the ability to djinn storm, and some sort of +50% everything buff, and hyperactive-something or other (her biggest nuke...which thankfully isn't huge, but if she does the everything buff right before, it might single-handedly cause you to lose the entire fight), oh, and she starts to move ridiculously fast. Sword of rapture PARTIALLY works on her at this point, it will get rid of her speed and mag buffs (I think it was mag, maybe mnd, I forget). But leave the rest. Even without the speed buffs though she still moves much faster at this point anyway.
Regardless, you may want to just farm floor 20 trash over and over until you get 1 of each rare drop from the enemies since you will have to farm them anyway after you beat the final boss for a star.