One time that it is absolutely essential to have a backup savegame is just before you start any promotion dialogue. It is very easy to mess these up, and you don't get a second chance at them.
Also, two of the promotion quests, those to Priest and Lich, cannot be completed unless you actually promote characters to those ranks.
My two preferred parties for this game are the mixed party of gladiator, assassin, priest, and lich, and the pure magic party of druid, priest, mage, and lich. The pure magic party is a little weak in ranged combat (although the priest can use the best bow, and a lich can reach Master in thrown weapons), but a fully developed druid with GM in Unarmed and Dodge is IMO the best half-and-half character in the game. He has decent spells, and he's a very bonny melée fighter.
It's a great pity that the game was never properly completed. IMO it had the potential to be one of the truly great Might & Magic games.