It`s as sdshadow said, mystic ritual is almost useless. Keep it on 1 lvl so asap you can pick some undead unit you use it in end of battle before killing last enemy. It gives insignificant exp comparing to normal attack or disurraption ray/lightning attack.
Get some lvl`s on vampirism/regeneration for asmo and jump in enemy troops because he get riciculous amount of exp being hit, retaliatting and attacking also, while with rest of army pick up chests and mana balls (good units in start are couple of ingusitors to generate rage for making walls/digging chests, or archmages to get guardian angel medal). Later use mana to get other medals fire mage/iron warrior/holy knight depending of your hero class. And casting pugmy/doom on enemy gives more exp for asmo no idea why.
Use asmo to kill most of enemy troops, and he`ll get more lvl`s then your hero before clearing debir and later from verona he can get 300-400 exp per attack in,same for being attacked and with unstoppable skill, can get 2k exp per round which is sick. Hero on other hand get 1500-2000 exp per battle in verona. There asmo will be at least 5-10 lvl ahead of hero. But this get boring very fast
