There an amount of xp determined by the monster strength and your party strength - call it x.
If you have no heroes, each unit gets x/2, and henchmen get x each.
If you have one hero, that hero gets x, henchmen get x each, and non hero units get x/2
If you have more than one hero, the heroes and henchmen get x/(number of heroes), and non hero units get x/(
(number of heroes))
They have changed how the first stage is calculated, as compared to fallen enchantress, so that beating monsters stronger than your group does not provide as much of a bonus as before.