How many non-mindless undead/chaff are you facing? Mind Burn, Soul Slay etc can't affect most undead, so they'll go to non-mindless commanders, thugs and chaff first. You might be able to defeat his armies by killing everything BUT chaff, first.
After a couple of times I tried this trick he has filled his ranks with some starving archers ... what a poor life that of a human recruit of LA Ermor.