How did I do it?

* never leveled up any character
* used the well in Kriegspire for 30 temporary lvls
* collected lots of horseshoes in castle Kriegspire (2 times) and on overworld maps - so my sorcerer could get all schools to Expert and water magic to Master (for Loyd's Beacon). Thats also why it took me "so long" to finish in game time.

Did you ever try something similar? Do you have any other challenge run ideas?

I personally like the solo approach as well (let all but one character drown in beginning of the game and never revieve them). You get loads of XP like that (times 4)

Another one I've thought of for MM6 or later is to not spend any skill points. In other words, you need to beat the game with only your starting skill levels (though hirelings are allowed).

I'm more familiar with the older games, so here are some ideas for them:
* MM2: Kill the 3 Cuisinarts without any prior battle victories. (In other words, your 1st victory, as reported by the game, must be against those Cuisinarts.)
* World of Xeen: Beat the game at level 1. (Same as the challenge you mention, but for an earlier game.)
* World of Xeen: Kill the Mega Dragon (on the Darkside) at level 1. (This has, apparently, been done.)
* World of Xeen: Get the Goober and Super Goober titles without first getting the Xeen Slayer Sword. Good luck with this one!
* Any early game in the series: Beat the game without the Cartography skill, without using Wizard Eye (and without Eagle Eye if playing MM2), and without any maps. In particular, you aren't allowed to make your own maps either! Is your sense of direction good enough? (I've done most of Bard's Tale 1 this way; the 4th dungeon is harder to navigate than any part of any Might & Magic game.)
* Probably any game in the series: Beat the game without using physical attacks. This means no melee attacks, no arrows, and (for games that have them) no guns. Any enemies you kill must be killed with magic. (For MM4, use of the Xeen Slayer Sword against Lord Xeen would be permitted, but using it otherwise is forbidden.)