I chose the Griffins as the starting bonus, although still have the impression it didn't matter much.
Godric should just follow the road to the north (if you start with resources, you'll be able to buy a minor artifact at the artifact merchant's along the road), capture the town and free Lady Isabel from the prison nearby.
The Inferno heroes (with armies of various rosters and strenght) will keep spawning mostly in the south, with your only town as their highest priority (only one demon lord strayed from his path in order to wander in the east). Demon lords usually arrive through two one-way portals (one in the south, the other in the east).
Therefore in the beginning always keep Isabel near the town. Build up according to the resources you got (I played at normal difficulty and was able to claim most of the mines too).
The creatures guarding the neighboring mines and resources won't put up much of a fight, not even several Bone Dragons guarding the gold mine just a few steps northwest of your base.
There are two subterranean gates (the southern one guarded by a stack of infamous Death Knights), but the underground is insignificant story wise. While Isabel could easily claim the treasure, Godric as a scout would merely waste his time.
I also revisited the artifact merchant to buy a few random artifacts for Isabel, but the purchase proved to be of no importance.
Defeat the creatures in front of the obelisk in the woods in the middle of the map, though. You will have to find Grail (Tear of Asha) in order to win the scenario. Meanwhile, send out Godric with a small army to find the other two obelisks and reveal the puzzle map. One obelisk is in the upper right corner, the other in the lower left corner of the map. I understand the ultimate artifact is placed randomly each time, though. In my case it was located south of the observatory in the central area (next to the road, one square south of the bush). Take the Tear of Asha to the town and build the structure. The heroes will gain a level each, too.
The text message says you should send Godric to meet King Nicolai alone (while Isabel would keep defending the town), but that might be a mistake. In my game a week of the Devil occured and a stack of two Devils blocked the path in the southern mountains. Luckily, I had sent Isabel to clear the road.
In addition, the Inferno is close here and two weak demon lords would have attacked Godric later on as well. I guess I should have given him an army, but decided to keep all the vital forces (aside from two stacks of Assassins and Hunters that had joined me) with Isabel. Thus it was she who flagged the garrison in the lower right corner of the map...
...but only Godric can trigger the final event and win the scenario.