Mobs like dragons, demons, bosses and soo on (including strong mobs like cave bears and stalkers) must implement minimal ensured XP values, no matter how many units lost, relative army power, number of champions and soo on.

This value can be dependant on power ration of creature.

If power ration over 1000, ensured xp is 50%, this means that 3rd hero wont drain any xp from combat with such unit. 

If dragon have MEXP of 200, no matter what, all surviving units will recieve 200 xp from combat.

Bonuses and penalties will be calculated from this value individually, champion with -25% xp wound will get 150, with 50% bonus - 300xp.

Ever if army entirely consists from heroes, minimal xp value is ensured.

If normal combat xp is 300 and ensured xp is 200:

1 champion  - 300

2 champions - 200, not 150 due minimal ensured xp.

This will make fights with powerfull creatured rewarding and will encourage sending champions on such combats to lvlup everyone fast and atonce.