Its worth doing the Prophet and Oracle quest if you haven't. The Oracle's Healing is much better for normal training but the Shaman's Noble Sacrifice is much better for PvP purposes. The Oracle quest is quite difficult with 2 extremely tough bosses at the end. Even with a group of 3, you fight a single boss and his group but the boss can wipe out 1 mercenary a turn (or more) if you aren't properly equipped or prepared for it.

Level 75-80 (Sunken Machine Shrine):

The Sunken Machine Shrine has a nice set of quests. They are fairly simple and straightforward. The only difficult part is with the Machine Beasts. The blue ones can attack all targets for a good amount of damage and a large group of them can be lethal. The best place to farm here is on the second section. The Machine Fighter type mobs are weak and easy. The only thing to watch out for is that they self destruct. The first floor has Crystal Guards that attack like spears so your middle row will take heavy damage. If your middle row is tough, they aren't bad. The third floor has Machine Arachne. They attack like archers and that is undesirable. They're still weak, but the Machine Fighter types on the second floor are the best overall. They also go down with 1 Judgment Scroll at atlantica gold level 80 and it is very possible to scroll until level 90 there if you wish to do so. The mini boss here is primarily the giant mummy. Compared to the other ones, it is weaker and not as much trouble. The boss here is the Machine Goddess and it is unlike anything you have faced before. If Tiamat got powered up several times over, you would have the Machine Goddess. I recommend you obtain level 80 stuff in the Shogun Castle of Death before returning to face her. Either that or get all your mercenaries the level 80 upgrade. It is possible to skip this dungeon entirely altogether.

Level 80-90 (Shogun Castle of Death):

This will be by far the most grinding you will need to do. There is a 10 level gap between this dungeon and the next and the dungeon here will only get you to about 86 so you will need to grind heavily to get to the next dungeon. There are 2 sets of mobs on each floor. You will need to be careful because 1 group is much more powerful and is meant for the Cannoneer quest so they act like shadow dungeon enemies would. The Ashigaru mobs on the first floor are fairly easy but you need to watch out because they are Spearmen and Gunners. That means if a battle lasts for more than 2 turns, expect to be stunned. Otherwise they're easy. The second floor has Ninjas. They are trouble and use a powerful poison. Avoid them if possible. The third floor have the various Dancer and Samurai mobs. The Dancers are weak and easy. There are 3 types of dancers but they're all easy. Its nice to farm them since they have good drops for crafters. The Samurai mobs are a bit stronger but just as easy. The Samurai spam Dark Seed and the Kagemusha can spam Hex of Darkness. If you're not careful you can get Hell Fire used against you. Otherwise they're easy. Chances are you'll be farming here until you can get to the next dungeon.