Depends on the units INT score and the spell to cast. Most NPC have a low INT score. It takes some doing to get them bumped up enough to use any good spells.

Indeed, it does, but the units in question don't even have the Cast Spell button visible on the toolbar during Tactical battles, only on the Strategic map.

Hmm... I thought Wisdom was removed entirely? Worth a shot, I guess...

Giving Wisdom didn't work, as I thought. The Wisdom stat is completely gone, it does not even have a UnitStat definition anymore.

It isn't something unique with the Imbue Champion spell either, same problems occur if it is cast on a non-Champion unit (after being modified to allow it, of course). Furthermore, giving every Champion starting Essence (thank god for find/replace...) lets them cast spells right off the bat, no problems. A carbon-copy of the Imbue Champion spell also works properly when cast on Champions, but not on other units.

My guess? A unit needs to be a Champion before it will even check to see if the unit has Essence in Tactical Battles. Or a bug. Oh well...

Hmm... thinking on it, I don't suppose anyone would know what exactly causes a Champion to be considered a "Champion", would they? Maybe that would fix the problem, although I suppose that has a chance to create other problems...