There are a lot more ways to kill him, than using the cheesy scorpions, or a frost weapons. . .
The thief Curse, can lower his resistance to spirit magic, fire, poison, and electricity. This opens up a bunch of options. You might have to cast it a few times, but it'll do it's job.
The thief blind spell, if it's of sufficient level, is quite effective.
If he runs too fast for you, you could always use the Wizard's, Limbs of Lead.
Heck, with the thief curse, you can do a ton of things to any enemy. Polymorph, freeze, poison, electrocute, barbecue, slow, blind . . just to name a few.
Think a little outside of the box, or in this case, the scorpion trap container.
EL
p.s. But, if you love the scorpions, to each there own. It's a GREAT idea, but they went a little overboard for my tastes.