I think I understand why boss def/mnd is so high in many cases; well first of all it's because they ended up giving the player several too-strong options that let you utterly demolish bosses that don't either have extreme HP or extreme defenses when you put the strongest characters in gambler builds. But anyway, the other reason-
It's because you actually do have a lot of ways to deal with it. You have armtwisting to combine with debuffs to debuff even highly resistant bosses without too much issue, or using Reisen and/or Hina to similar effects. There's now a large array of characters that can ignore a significant amount of def or mnd (even Iku!), and Monk has an option that can sometimes get the job done. Some of these bosses might be vulnerable to PSN, although I think a lot of them do resist it... (It's so much harder to use Wriggle in this game. I wonder if PSN even does alright damage still in postgame?)
But my team actually doesn't carry any of those, I didn't have -anything- until I got Rumia recently, who sure is important in randoms now. That's more my fault though I guess? As for the "overlevel and power through with Nitori/Flan/etc sheer numbers" option, it's there, but I was more or less immune to all of the boss's attacks WAAAAY before I managed to start hurting it, so obviously it wasn't the intended solution. I think having a counter for these bosses of some form in your party is really important, and thankfully there's a good variety of choices for that.
Hmm.... any level recommendations and suggestions on beating the last boss, it's been awhile since I picked up LoT2 and plus I want to get moving to prepare for Special Disk.
So long as you've actually spent your money in the Library and not stockpiled 1m+, the challenge level should be fine in terms of surviving everything except perhaps the final phase. However, you've got to figure out how you're damaging it through it's DEF/MND; you can debuff it off with ArmTwisting or Hina/Reisen or at least using "Eyes That Perceive Reality" characters, or you could use straight off def/mnd ignoring characters. OR, you can do what I did, and use Nitori and Flandre to just brute force through, which works because Nitori is OP and never has to be switched out ever even in full ATK builds with her crazy nuke potential, and it's weak to DRK so Flan works wonders.
In other news, seriously, if you were going to banlist anyone, banlist Nitori, as you pick up better and better gear she just gets stupid good. In other news again, early in the game her nuke didn't seem particularly good... so I wonder if Youmu's nukes also scale a lot better than they seem when you first get her. A lot of people comment that it seems kind of worthless, and I haven't heard anyone's opinion on Youmu later in the game.
e:Daaang they buffed Momiji's stats all by so much in the 1.20x rebalance. She must be an awesome tank now. You need to be to compete with the gimmicks the other guys get, of course, but daaang. Kogasa, Rumia, Cirno, Minoriko and Parsee were the other who benefited the most, even though all Parsee got was a MND tweak- in that her already ridiculous mnd got even higher. Shikieiki did get a high DEF boost, and Meiling got a significant ATK boost. Most (all?) other tweaks were minor.