The first thing you must do is to build a unit with all of your strongest people. There are no limitations to this unit except that it has to have one of those eagle dudes so it can move really fast. At the beginning of each level, have all of your units except for the unit you just made take the quickest route directly to the boss's castle. Forget about liberating cities and temples. The unit you just built you should move to the boss's castle too, but it should take a different, longer route as it will be travelling faster.
The bad guys will be so occupied with your big army marching straight toward your castle that it will not notice one little unit wandering around the back. After you beat the level, go back and liberate all the towns you missed.
Bonus Fight
For a bonus fight against a huge enemy force, start a new game and enter your name as FIRESEAL.
Cool
Enter as Name 'Music/on' to play music tracks.
Faster Gameplay
For a faster game, start a new game and enter your name as GOTOHELL.
Make Evil Units Quick
NOTE: characters have to be level 5 or higher for this to work. Do you have a wizard with a ali. to high to become a mage? Or do you have a dragon you want Red or Black but his ali is to high? Well here is your solution. Go to Sharom Border. Have the characters you want evil in a unit. Have it either be a high sky, or a mountain unit. Send it on a course through the mountains south of the rebel base. You will randomly encounter level1 dragons. Slaughter them to quickly drop ali. NOTE: charisma will also drop, this can prevent human characters from promoting to new classes.