Here is the official "COMPLEXITY" intex / table of locations:
Greenwort 5
Verlon Forest 15
Marshan Swamps 25
Arlania 15
Western Islands 25
Eastern Islands 25
Upper Hadar 35
Lower Hadar 35
Ardan Peaks 45
Taron Mines 45
Mehgard 45
Magic Valley 55
Great Wood 65
Valley of Thousand Rivers 55
Roots of the Life Tree 65
Haas's labyrinth 95
Grey Heathland 75
Death valley 85
Demonis 95
Murok 95
Please look at this table only on high level - these numbers do not directly describe the battle difficulty - as the difficulty is based on another aspects as well (different troops/different combinations/different tactics etc.) but still it shows the fact that in Demonis you may expect much higher challange than in Taron Mines, and that the easiest fights and lowest rewards are Greenwort)
Also you may see that Verlon Forest and Arlania are not as difficult as Marshan Swamps - so don't seek out battle too early in Marshan Swamps... even if you can access them as easy as the other two locations... (maybe if you play easy - the difference is not as visible) but I remember playing impossible and finding it impossible to win against 600 venomous spiders with 8 archers, 12 swordsman, 35 peasants, 6 priests and 12 lake dragonflies
... with my puny fireball that did 300 DMG in average
P.S. maybe my good friend that gets so bored by BLAST BLAST tactic would win it as he is da best mage ever