Oh, I hadn't registered it was you who posted about arrow fend last time...
I now had a similar problem in the same game. My communion master scripted [communion master, undead mastery (x4)] to take out the huge opposing undead army (5000'ish) decided to cast bone grinding instead...
Then, for some strange reason, after his script ran out, he decided to use the gems he had left because of using the cheaper bone grinding, to cast one undead mastery.
We're late end-game now, managing freespawn & insanity, dozens of ritual spells & forgers, and especially carefully scripting huge armies takes a couple of hours by now. (hell, watching that one battle we're talking about start to finish took almost an hour) After seeing all that effort blown away, I have to say my motivation to start on the next turn is very low now...