1) If you're playing as a hero and want to capture mines, first capture the mine itself, and then collect resources around it.
2) Try to develop expert-level Earth magic and other spells (the more, the better).
3) Don't forget to upgrade your Intelligence.
4) Expert-level Wisdom is a must.
5) Don't forget to use the personal grudges of monsters (Titans vs. Black Dragons).
6) If possible, wait until the enemy units approach, and then attack (I mean Titans and Master Gremlins).
7) Annoy your opponent with a lot of useless hero-partisan units (if you have the money). They should run around the enemy territory and capture mines. The enemy will either want to eliminate them (which is likely), or split their army (one part will stay with the hero, the other in the city).