[ivra]

I did some research on enemies with a single type of unit. For instance, all peasants. It allowed me to rule out a lot of things that do not affect the XP you gain after a battle. Notice: I have just begun and I have no idea if what I have found so far is true for boss fights and enemy hero fights. But for a no hero enemy fight I have found the following relationships:

First: What does NOT affect the XP you gain:
- Your Level
- Whether or not you use your Pet Dragon
- Whether or not you use Spells
- The number of troops you use (single stack of 5 full stacks, or any other combo)
- Whether or not you pick the chests that are preset on the battlefield.
- The game level (easy, normal, hard or impossible)

My conclusion is that the XP is based only on the unit you fight, and that it follows the following formula:

XP = 1 + FLOOR(NoOfEnemyUnits / XPFactorForThatUnit)
(FLOOR(X) means rounding X down to the nearest integer)

What remains is to figure out the XP Factor for all the units in the game. To get this far I did some simple battles in the start of the game as shown in the list below:

Unit............ Quantity ... XP .... Unit/XP ... XP Facor
Peasants........... 185 .... 16 ...... 11,56 ......... 12
Devilfish.............. 14 ..... 3 ........ 4,67 .......... 5
Devilfish.............. 15 ..... 4 ........ 3,75 .......... 5
Devilfish.............. 19 ..... 4 ........ 4,75 .......... 5
Devilfish.............. 28 ..... 6 ........ 4,67 .......... 5
Thorn-Hunter....... 97 .... 14 ........ 6,93 .......... 7

The formula works so far, and the XP Factors seems to be:
12 for Peasants
5 for Devilfishs
7 for Thorn-Hunters
(High-level units like dragons will probably have a XP Factor less than 1, which means that one dragon will give more than 1 XP)

It is possible to find all the XP-factors by playing the game as I have done here, but it would be much more efficient if someone with inside knowledge could post a list, and also verify or invalidate what I have written here.