Hello,
I've just finished one battle level strong in which my spell caster (army strong too) just wiped out the whole squeleton army (6 or 7 brand new units) with one well placed blizzard spell covering a large area... and for that my hero just got 9 XP !
It seems to me very low to have won a level strong battle by myself ! So I guess there's likely is a bug when calculting XPs gained for targets killed by blizzard.
Thanks