You will need to farm the Kagemusha type for the Black Keep Key. Without it you cannot face the boss of this dungeon. Nobunaga Oda is a very tough boss. He attacks all targets in a column and his attacks are powerful. His area skill is also one of the strongest. Its recommended that you don't face him without either scrolls or a fully upgraded and equipped level 80 team.

The problem here is there is a big gap between this dungeon and the next. One of the things you can do is the Inventor quest if you have a level 50 Princess or do the Cannoneer quest. The Cannoneer quest has mobs with very high attack but low HP. If you can deal with this then the quest is fairly easy. The Inventor quest has a nice place to farm in the last section of the Hidden Machine Shrine. Some of the enemies are immune to freeze here. There's also several side quests but either way you will need to end up grinding up to the next dungeon. The first main character only weapon also appears through the quests here, the Cursed Sword Muramasa.

Level 90-95 (Lost Volcano Valley):

After the previous few levels of mindless grinding, you end up at the Lost Volcano Valley. There are numerous side quests here and the next set of main character only weapons start appearing. The enemies here start mobbing more often and start using more area skills. One of the raptors use a poison and AP drain damage. Lots of enemies spam different area skills. The best place to train here, has to be with the Flame and Fire Spirits. They are weak but give decent experience. You might have trouble if your front line is composed of axe users since they spam skills. Their drops aren't that great but they're very easy to train on. If you wish to do so, the Flame Giants and Dragons are also good to train on but they spam an all targetting weak meteor skill similar to Beast Summon. Its not strong, but it can quickly add up in terms of damage. The first miniboss you encounter here is the Broken Beast Machine. Its a powered up version of that evil spamming mob from the Sunken Machine Shrine. The second miniboss is the Yoruichi which is one of the Dancer type mobs from the Shogun Castle of Death. This one uses high damage to a single target but other than that its fairly easy. There are several bosses in the Lost Volcano Valley and they require keys, but you get the keys through quests and you can get more through farming. They're fairly easy compared to the previous bosses you have fought.