I think heroes should always be leveled up to the maximum. Before playing, I read forums and websites on the topic. And even so... I replayed the battle with Kujin 5 times. The rest I passed without any problems. The final battle... Well, I leveled up Zehir to master light magic + blessing + protection + some skill that guarantees that a spell (slowing down instead of speeding up, for example) may not work. I learned Divine Retribution (I didn't find any resurrection spells in any library). Divine Retribution helped a lot.