Ehm.. I'd say it's maybe harder than the lich quest... In lich lab you can atleast own everyone with Turn undead (except eyes), but those Terrors are really bad - they put down my gladiator with ~1000 hp in a few hits - they do not "turn" of turn undead spell even at GM lvl...

The easiest magic promo quest is druid - practically no fighting, only some easi Gezzampts and only 1 tough battle (tough at this moment of game) with 4 yetis.
The second is Priest promo - temple in frosgard is not so very difficuilt... Magreebs on the was to Monastery are not very easy, but anyway they could not be even compared with those terrors from isle of aches...