I myself have used Lord Mazdamundi in a battle against Dark Elves. I kept him near the middle of my army. so that my units could use his leadership. I took the Lore of Light, and used it's healing properties and destructive 10+ spell. When a frail unit of corsairs were within range, I charged them with Zlaaq, and the unit passed their Terror test. The impact hits wiped out 3 of the corsairs, and Zlaaq killed another. The opposing unit, as far as I can remember, failed to wound back.
We only played 3 turns in that game, and to my knowledge, Mazdamundi didn't perform as I would have liked him to. Personally, i would rather take a custom Slann with four Disciplines; Higher State of Consciousness, Soul of Stone,The Focused Rumination and Transcendant Healing OR Unfathomable Presence. For Magic Items, I take Cupped Hands of the Old Ones, Divine Plaque of Protection and the Plaque of Tepok. This gives the Slann a total of 5 spells, instead of paying 50pts for the Focus of Mystery, which allows the Slann all spells from a single lore. I also make him the Battle Standard Bearer with the Sun Standard of Chotec.
This combination is very good, I think, at protecting the Slann. his extra Power Dice may cause a Miscast, but Soul of Stone and Cupped Hands of the Old Ones can combat this AND make it quite nasty for your opponent. Put the Slann in a 20-strong unit of Temple Guard, and he will be nigh-unkillable. Hope this was helpful :-D