Since Paladin/Wizard/Sage has access to all spells, I would go for the Viking or Rogue. A neat thing is, the Sage would then have to get two indirect white spells and nuke spells (if you want all spells). So the Paladin has to do all the healing. I bet a smart player can manage doing that and actually breeze through the game by using the right spells at the right time (and running away, hah).
It could also be fun finding out who makes a better leader: Rogue or Paladin. Also: since you know your way around now, you can 'steal' the Rogue's best weapon early on and have fun with that!
I've taken note of what you say about bosses. They all have harder attacks while the fight progresses, so the timing of buffs and attacking is critical. You get extra time against bosses with SHEL and SHLD, true, but you might not still have those spells by the time you make it to the boss. If you do, you probably went back and forth a lot in the dungeon. Maybe I'm wrong about my assumptions.
My philosophy is that if you take the time to grind and search out the dungeon, the boss is going to be quite doable; that's the reward for being well-prepared. If players want to head right into the boss, that's also possible, but the odds are more against you.
Parties with access to buffs also take longer to deal with trash mobs.
Finally, I want the game to be beatable by any 4x different character (no Fools) combination, which means I bosses have to be beatable by a team with Thief, Ninja, Warlock, Viking.
I'm quite happy with the balance of the bosses, but it's good to hear what people are saying and I'm taking notes for further projects. So sincere thanks!
Oh yeah, I'm happy with the Monk hitting so hard after two buffs at level 38. He's a subpar class up until the later phases, so I'll let him enjoy being the best all the way at the end : )