[ologm]

Fever fetishes might also be an option. You've got plenty of nature income and the fetishes supply the fire gems. You also have rejuvenating researchers to juggle the fetishes around on. Remove fever fetish when a casters health becomes too low and switch it to another, allowing the first to heal.

A couple of harpy scouts juggling a bane venom charm can also be used to damage opponents. I did some tests a while back and bane venom charms seem to disease 5% of an enemy army of normal troops each turn.