Fighters are kings of the first level

They level faster than Samurai and will get additional attacks faster. Create three of them and use the back three slots to carry around your mages and priests that you are leveling for real heroics. Grinding the 1st level (including Murphy) is the first stage in defeating the game. Once your Fighters can handle Murphy on their own, they can take just a fourth character along to increase the experience earned for that additional character. Once a single fighter can handle Murphy, take just one of your leveling characters along to increase the gain further. I hope you like the look of Murphy, you're going to be seeing a lot of him. Do this until you cannot stand to anymore, then venture into levels two, three, and four to get the necessary items and access to the second elevator with your 'real' party.

The Bishop is not a party member, he is a tavern dweller

Certainly create a Bishop. Have him hang out in the Tavern while your real adventurers do heroic things. When they come back to town with items in need of identification, clear a space for him, go into the dungeon and let him identify away. If he gets cursed, either pay his bar tab or kick him to the curb and create a new one. If you want a character that can cast both Priest and Mage spells, then create a Mage and switch him to Priest at 13. Do the same with a Priest, switching to Mage at 13. This will cost *far* less experience than trying to get a Bishop to a level where he can cast all the spells.

Lords and Samurai are not equal

Both require key items to really shine, but the Samurai level a little faster and will be better at killing things than a Lord. A Lord in position 1 will probably be a good damage soak though and you will be glad you created one if you get those cool loot drops. Want a Lord that can cast Mage spells or a Samurai that can cast Priest spells? Then you will start as the appropriate caster and get to level 13 before switching to your desired class. The only reason to not do this is because you hate grinding on Murphy. You can never have too many spells!

The Thief is terrible

Sure, he can detect and disarm traps, but he gives up everything else, including staying power and combat ability. You will have enough characters with Priest magic to handle detection magically once you start caring about the loot in chests. If you want a Thief by that time, then start him out as a Mage or a Priest (or do both if you really, really like Murphy!) and then switch into Thief. If you hope to become a Ninja, then you will need to start out evil. If necessary, take the character into the dungeon and "find" him with your good aligned party.

The Party with the Plan

There are a few variations of what you can work toward before heading into Level 9 and 10 of the maze.

POSITION 1
Lord (that started as a Mage) or Samurai (that started as a Priest) or a Priest (that started as a Mage)

POSITION 2
Samurai (that started as a Priest) or a Priest (that started as a Mage)

POSITION 3
Thief (that started as one or two casters and maybe became a Ninja) or another Samurai (that started as a Priest) or another Priest (that started as a Mage)

POSITION 4, 5, and 6
Priests (that started as Mages) and/or Mages (that started as Priests)

Once you have your chosen lineup established and they have all class switched at least once (or are close to doing so), you can start to tackle levels 5 - 8 of the maze. There is nothing important on any of those levels, however, except maybe the occasional minor equipment upgrade from a chest. Make sure your Thief is ready (if you have one) before worrying about chests though.

Once the back row has all the spells and your front row seems resilient enough, you are ready to tackle the final two levels. Grind both of them for as long as you can for the rare loot.