[K]

The easiest AND most powerful tactic is to get a Fire mage and cast Flaming Arrows. Combined with your Longbowman, this is a devastating combo.

Make Fever Fetishes and pass them around scouts so that your forward armies always have fire gems to cast the spell.

If you haven't gotten Fire mages, you'll need to make one. Empower a young guy(since Fire magic ages casters) and make items like Flaming Skulls. Make sure to script Phoenix Power, and don't forget that with an extra gem you can cast a spell one level higher.