even if Zakh has 100 Phoenixes left, they will still single-handedly defeat an army that outnumbers them 50 to 1. On the other hand, the map offers many ways to develop heroes; theoretically, the Death Knight strategy described above, along with a couple of leveled-up Black Mages, could severely cripple him.
However, to make him completely invincible, you only need to give him protection against black magic and remove the Death Knights from the map.
In other words, a couple of Black Dragon units, and no units that are susceptible to hypnosis, etc.
(Personally, I think a completely invincible Zakh would be uninteresting; it's much more fun to try to kill him.)

BDP? = Battle to the Last? It's leveled up, but how does it affect things? After all, I'm the one attacking them, not the other way around.

My suggestion is to add a bonus when equipping both super swords from Might and Magic III.