Sometimes when I'm fighting powerful mages and try to target them with offensive spells i get a message saying that enchanted or invisible persons cannot be targeted, not even with pierce magic, breach or dispel. So how do I make them vulnerable again?

True sight should work in all cases. Depending on the spell that was used by the mage, detect invisibility or illusion may work as well. In theory, the thief skill detect illusion should work, but I haven't had a chance to test this out.