Suggested fix for targetting units in armies with strategic spells.

When you click on a stack after selecting a strategic spell, a box pops up with the individual units in the stack and boxes below them.

left-clicking selects a unit

right-clicking inverts selection

a Done button will cast the spells.

This is the system used in the Age of Wonders series, it worked perfectly well there, and I see no particular reason to re-invent the wheel.  This would probably take someone a day or two of work, but would solve the problem entirely.