Strands of Astral Power. Worth it?

[Dedas]

But you shouldn't expect a spell that relies on a simple magic resistance check to work very well on MR 17-18 units. It's very logical. If you want to lower their MR you do that by magic scale (not in this game of course), penetration artifacts and high magic level. I don't know if the latter works on Strands of Arcane Power but magic scale sure does.
The spell doesn't suck, but it wasn't the best one to cast in your situation.