After much more trys I now favor the following strategy:
(first I must say that I play on the smallest map with 11 enemies on the 2nd highest word- and enemy-difficulty)
1. I take Tarth as race (the double attack is awesome); my hero becomes a scientist with earth and life magic, 500 gold, +1 life/level, escape from battle and clumsy; race traits are fortune teller basin, fast, accurate and no armor. Reasons: I want to raise research as much as possible, life magic gives call of sovereign with +2 growth to the city and earth magic +1 material, the troops should hit (double attack reduces accuracy) and attack fast (who attacks first wins, armor reduces speed)
2. a ) I settle on a 3-3-3 terrain (if possible I found a 2nd city): Reason: If I have luck and get heros with air and fire magic I can produce very strong troops with +5 to initiative and +5 fire damage. The goal is to raise the city as much as possible to level 4. I only make conclaves with +1 essence and +2 crystal income. Crystal is rare and necessary for magic items.
2. b ) Alternative if there is no 3-3-3 I settle on a terrain with high materials like 2-5-1. Changes to my strategy: I must produces mages (archers are too weak without the firedamage) and I build no alchemist laboratorium first because I need the crystals for the mages and productivity is already high with 5 materials.
My experience: Alternative b is the better one: 2 grains are enough to raise a city to level 4 (more is not necessary). Remember that you can cast a spell which gives an additional grain! At the end you have either 3+1+5=9 materials (on 3-3-3) or 5+1+2=8 materials (on 2-5-1). The difference is not great. But with 2-5-1 you are much faster to have all necessary buildings for food, research and production (due to the fact that you can build the food buildings much faster you can compensate the fewer grains). At the end I am about 20 turns faster to be able to build mages! And with 2-5-1 I must not rely to get a fire hero!
3. Taxes are set to 0. I spend the money for treaties of peace so I must not build troops and can produce buildings. The sovereign and the heroes become leaders. The heroes stay in the city to reduce unrest. The sovereign learns the arruracy/speed tree to ensure that my troops hit.
4. I go for the civilisation tree on the bottom first to raise production. I must have the great mill (+25 % production). I save my crystals to get the alchemist laboratory. This building boosts my production (+1 material per essence). In the last game my prodution raised from 100 to 213!
In the last game I reached this phase at turn 113.
5. Now I am ready for producing troops. I either produce archers or mages (depends on having fire magic: if yes => archers). When I reach this point I am unstoppable.
Does anybody have a better idea?