I also used auxiliary heroes to visit all the places that boost stats, and only then completed the mission. It took a little longer than it could have, but in any case, Talannar helped me a lot with the Greater Vampire.
Yes, in the 5th game, I remember sending him to the Necromancer's castle, and he worked with the undead. Maybe I should have visited more places, but... I didn't:smile33:. Or rather, I visited the ones I came across along the way, and most importantly, they all learned the full set of spells.