The only things that reduce a monster's mana are:
- Using it to cast spells
- The Mana Burn specialty ability
- If a monster walks over a rune of mana, some of its mana will drain into the rune
- Disenchantment attacks (I think the only way a player can get these is from Chaos or Balance DSM)
- Chaos attacks have a chance to set monster mana to a random value (so if it's more than half gone already, it's more likely to increase than decrease)
So basically it's just a matter of waiting until it runs out.