I took out the 9F optional boss about halfway through my exploration of the maze that starts at 10F; and went back to take on the 8F boss after finding the relay point.
Also, yeah. The Plus Disk is 21F-30F, but be ready for the necessary grinding to spike like a mofo at 30F. Just looking at the wiki data, the recommended level for boss 1 of that place is 300... the recommended level for the uber boss, however, is a whopping 600. So... yeah. And that's -with- maxing out your affinities and boosting your stats to obscene levels and stuff. The only reprieve you get is that the enemies on that floor actually make grinding to that ridiculous level feasible... it's still gonna timesink you like a Dragon Quest game does, though.
Also, NeoSerela, those two characters are -great-.
The F7 character has two moves that are wonderful. One's a row-target move that has the Death property. Damage is alright on it, but most importantly, it can randomly (And surprisingly frequently) one-shot multiple enemies by inflicting Death. Not so great for bosses of course, but... for bosses, you have the spell listed after that. It can inflict four stat downs, PSN, and PAR in one attack. Even if the attack power on it is crap, its support capability against bosses is ridiculously effective, plus the character's HP is so far through the roof it makes up for her terrible defenses because she just soaks up the damage anyway.
The other character you got is also really powerful; she may be very fragile, but both her ATK and MAG are very very good and she can use either (ATK for one target, MAG for multiple). Most importantly, though, her first spellcard is an extremely effective Paralyzer - it seems to hit more often (And with a stronger effect) than even Cirno does.
Put the three of these characters together and it's completely possible to keep a boss on practical lockdown while a slower nuke like Patchy or Y_____ nails em.