It's just a little hard for me to advise you without knowing the kind of stuff you do possess. I act like a pack rat in these games refusing to give up magical weapons unless I have more than I could use at the same time or am hopelessly encumbered, that gives me more options for these battles generally.
I think I've had problems with that Summoned Skeleton Chieftain before myself. In those cases a Haste potion is also useful (though you're probably using those anyway). It could be possible to attract Desther's attention, causing him to cast a spell then retreat out of the distance of whatever the result is, if you could then rest whatever he has summoned should vanish and he is more unlikely to summon another. Other than that, you should be able to recognise the spell animation for when he is summoning a creature - if you have any way of increasing your possibility to hit - Divine Favour, Bless, a wand of Magic Missiles, anything like that - use it in an attempt to disrupt his casting.
I think there's a kid on the bottom floor of Helm's Hold who acts as a merchant. You could buy a magical blunt/crushing weapon, that would require a lot less backtracking and be far more effective against any kind of Skeleton than a slashing/piercing type of weapon.[/QUOTE]
Ah, yes, the Bag of Holding vendor in the dungeon!
Now, if only he had a lot of good potions (healing, buffs, etc.) on him as well. :mischief: