If you want to be able to cast more powerful spells in battle, there are few tactics more powerful than Communion abuse. However, Communion does require astral (or being a Pythium communicant, which auto-casts Communion Slave w/o knowing magic, or to be carrying the appropriate crystals created by astral/earth mages) or Sabbath (which requires the expenditure of blood slaves, if memory serves). Communion works best if at least one Master has Nature-1 as well and there are enough communicants to be able cast Relief as necessary. You can get to truly sick levels of magical power with enough communion slaves... an astrally powerful master with numerous communion slaves, a spell focus and a rune smasher can be incredibly effective at Master Enslave.
Storing stuff on scouts is a good idea, especially for things like supply generators which don't directly help in combat. You can also use them to store gem generators (notably astral clams, earthblood stones) and shift gems over to your casters as need be, and if you're concerned about blood slaves being wiped out in battle you can hide and ration those the same way.
Regarding priests, it can be a good idea to script Divine Blessing once. Most of their casting isn't damaging, however, unless they're mage-priests who could be casting something more worthwhile.
Re: A question on AI and magic use (spell selection and gem usage)
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