The only spell that I've found to work reliably well against them is Implosion. My usual party (KAPRCS) contains only two characters with access to that spell (AS), so I try to retain some items that can cast the spell with which to equip some of the others.
I agree with your opinion that MM3 is the best of the Might & Magic games. A non-linear game containing a blend of continuous action (no boring bits, unless you want to explore all the seas in addition to the bits around the Isle of Fire), some cracking good puzzles and riddles, (not to mention the humour!) is very hard to beat.
It is SUCH a pity that the single-player RPG went out of fashion!