As Ermor, you want to spam Nether Bolts or Nether Darts at them until they are Feebleminded. This reduces MR by 5. It's evil, and it works, and I hate it.

Once that is accomplished, destroy at will with your favorite anti-construct spells.
If the Nether Bolt doesn't Feeblemind like the Darts (I can't recall for sure, never use it!), then some Black Bows can do the job just the same, and are cheap to produce.