I assume you left him as alone as possible while you killed his army? Malus is one of those LL that you either have to throw multiple characters at to kill in good order, or force the army losses on them. Depending on what kind of army you took against him (melee vs. ranged), that can be hard or very hard to make happen without significant losses.
Might check his magic resistance in both forms. Single target character attack spells like Spirit Leech get a pretty bad rep for good reason. But this is one instance where it might make more sense to just zap him into submission than wasting arrows (i.e balance of power) that don't hurt him or sacrificing melee units (also balance of power.) If you want to inflict the army losses retreat on him early, you have to lose as few of units as possible before you engage him, and while you're engaging him. Every unit you lose means that much more (really difficult to apply) damage he has to take before he'll break, even after his entire army is gone.
Malus is up there with Vlad, Grimgor some dwarven LLs and a few others that even in the AI's hands are basically not worth fighting with anything until the rest of their army is dead. If he were slow it wouldn't be that big of a deal, but with Dark Elf speed plus being very hard to kill and a decent melee combatant, he kind of demands an answer.