The idea here is that you can harass the Orcs and retreat if things get tough. The main point is to prevent the Orcs from inflicting heavy losses on the Undead army and to weaken them first.
Yes, I mean that in this replay, the Undead player got a bit lucky and managed to eliminate all the Orcs. Nevertheless, there was a chance, not to destroy all the Liches (there was no chance of that), but to prevent him from causing so much damage. However, the Undead player would have retreated, and the situation would have repeated itself in about five minutes. Thus, the Orcs' defeat would have been inevitable.
P.S. Here's another replay on the same topic, but against the AI. Maybe someone will doubt its competence?:D Even fewer losses. I specifically made the Orcs' defense so strong that it's insane.
And also. I generally used Cold spells to destroy machines, but I think that if there are no problems with machines, you can go for Light and take Light Magic and Divine Might, which will guaranteed kill a stack of leaders in 1-2 turns.