The biggest problem I can think of is that Teleport is *mandatory*; you can't beat Clouds of Xeen without it (unless you're willing to use the mirror to teleport you straight to the final boss). This restricts your choice of class to those that can cast that spell, which means you have to use a Sorcerer or Archer, unlesz you want to keep re-rolling shop inventories until an item of teleportation decides to show up.
Given the cost of the Teleport spell, and what spells Rangers and Druids can cast (almost every spell costing at most 10 SP (or 2/level for scaling spells), you would expect Rangers and Druids to be able to cast this spell, but for whatever reason that particular spell is excluded from their spell list. I have no idea why they did that, and I consider it bad game design (because of the spell's status of being mandatory), but that's how the game works. (I believe Jump isn't so restricted, so completing the Darkside should be possible without having to find such an item, but Clouds requires Teleport.)
A more reasonable challenge would be to complete the game with a full balanced party, but without using the trainers to increase your level. In other words, the only ways to increase your level are the fountains that provide temporary bonuses and the permanent bonuses that are rare until you reach the World of Xeen combined game specific content.
