- Defeat Worton.
You start out with six level 20+ heroes, and nothing on the map will be able to stop an army like that. Since this is the only campaign where you get so many high level heroes, you might want to keep them together, just for the novelty. But it's probably best to put the named heroes into one army and the two random heroes into another army. Then the named heroes can do most the work while the random heroes pick up and flag stuff.
Once you decide how to group your heroes together, head east. You should find two quest gates and a tree of knowledge. The first quest gate is optional. If you send a hero with three vials of acid to it, you'll be able to pass and visit a couple skill buildings. The second gate leads to one of the three neutral towns on the map. It'll let you pass if you have Desette and Kentaine in your army. Then, once you take a few steps past the gate, the neutral town will convert to your side. (The other two neutral towns will convert to your side on their own, several days into the scenario.) The tree of knowledge requires gems, so hit that as soon as you can afford the price.
Once you clear out the area near your first town (being sure to visit it, so you can set a governor and get Desette more spells), send your main army back to the starting point while your secondary army uses the portal. The portal connects the three towns that will eventually be yours, and your secondary army should be powerful enough to clear the areas around the towns and flag the nearby resource buildings.
Your main army should then head south into a fenced-in area. There you'll find an optional quest hut (defeat the Blue-flagged peasants to the west for eight healing potions), lots of troop buildings (peasants, ballistae, monks), and a blacksmith. Be sure to visit the blacksmith to get your heroes some equipment.
Then head south again, and finish clearing out the areas surrounding your three towns. You'll end up controlling several resource buildings, and you should even find another tree of knowledge. Meanwhile, continue to build up your towns, and go with angels at each one. You should be able to afford them in this scenario, although it might take a while to build all the altars.
From here on out you'll have to do battle with the three computer players, but notice that all attacks on your domain have to go through your southern town. So build that town up and keep troops there, just in case the computer decides to attack. (It probably won't.)
You can do as much conquest as you want. Teal is located to the southwest, and it is convenient because your southern town blocks access to that area as well. So if you take it, your southern town will still be in a position protect all of your other towns. And to get to Teal, you only have to kill a large stack of hydras. Since hydras are big, slow-moving melee units, they shouldn't present a problem.
Orange is located to the east. The path there is guarded by a large stack of thunderbirds, and thunderbirds can be a problem. But you have to get by the thunderbirds since the path they're blocking also leads to Red (Warton's color). The best way to deal with them is to cast mass slow and then pick them off before they can get to you. But if you don't have mass slow, try to let your heroes take the brunt of the damage, and keep your angels mostly out of the way.
After you defeat (or simply pass by) Orange's first town, you should notice the entrance to Red's lands. Red is blocked by a quest gate that only allows an army with Lysander to pass, so Red can't go anywhere or attack any of your towns. So feel free to defeat Orange's second town if you want, just to be thorough. Then head for Red.
When you capture Red's town, you'll find out that Worton has escaped. You'll find him to the east, past another quest gate. But the gate will only let Lysander pass, so make sure he has all the good equipment, plus assorted potions, and send him through. You'll find Worton plus some bandits on the other side. If you've been developing Lysander's combat skills, this should be an easy battle. You might even be able to kill Worton before he gets to you. Otherwise, just use potions of immortality and healing if you need them, and grind the battle out. Once Worton dies, the scenario (and campaign) will end.