You say "I tried Shrapmetal against some dragons, and one cast wasn't even enough to bring one down."
Did you expect it to? I have never yet brought down one of the top monsters (those giving level 6 treasure) with one hit from anything.
The weakest of the 3 top dragons, the red dragon, has 880 hit points, and I know of only 1 spell that can do anything approaching that and that's Mass Distortion. Unfortunately, it's a most unreliable spell and most of the time the damage it does is quite small. Also, it won't touch creatures immune to magic, and there are a fair number of those. The best I've ever done is to bring down an undamamged gold dragon with three casts of Shrapmetal, using casters with Dark Magic mastery and an effective skill level of IIRC about 57.