I didn't have any logistics or light magic skills. The unicorn just stole your spells, and I leveled up its speed from attack.
Yes, that holy strike of yours and the stupid ballista (I should have spent two turns on it) were the biggest obstacles, and of course, the speed... I also had artifacts to reduce enemy defense and luck, but I decided to keep them for myself. It's a pity that the defense and luck were reduced (only 2 luck remained, and the defense was at zero), meaning all the leveling was for nothing, and you can't run away with just one attack and defense, even though I had a decent army. Maybe I was doing something wrong because of my noob status in Heroes 5 (not in terms of leveling, I don't know anything about that... but in terms of combat, because I mostly played duels, and they've simplified these battles, which doesn't give you enough experience).
P.S. I still think runes are cheating because they provide extra magic for EVERY unit and on EVERY turn.