[thejeff]

They're undead: Undead Mastery, Wither Bones, etc

If they're guarding commander, they aren't attacking you, so kill them at range. Flaming arrows, any evocations that do magic damage.

If you have to close, use magic weapons and/or undead that won't get life drained.

Without knowing who you are and what you've got available, it's hard to get specific.