Enemies are bound by the same spellcasting mechanics as players. If you move behind them or otherwise out of their Line of Sight when they are casting a non-AoE spell, their casting will fail. In this way, you can force mobs to fail at spells like Void Thunder III, Fire III, even Water and Fire. This does not apply to spells that are AoE like Blizzard II, Fire II, etc.

Further to this, you can fight some enemies without taking any damage at all. Firemane in Halitali is one example. Lock on and strafe in a circle around it. Every single cast of Fire it attempts will fail because of the above mentioned mechanic.