The thing about magic immunity is, despite the name and description of the ability, it doesn't actually make you immune to all forms of magic. It just boosts magic resistance by an extra 100 points. Sounds great, until you realize that damaging spells (and all strategic spells, I'm pretty sure) completely ignore magic resistance. So while you won't be able to Slow those ophidians at all if you're in combat with them, you can pretty easily burn them to death with fire spells, or immobilize them with Tremor.
