Saying a god of death will auto-kill our troops is a bad idea, because it's not true. It means we have an equal playing field, because our life domain will counter act his death one, meaning he can't just obliterate our army, as our fire will counter his frost. In turn, this means we likely can't drive his people insane or spread horrible evil mutating disease that make you shit fire. Instead, it's a measure of troop quality, quantity, and abilities.
While I agree with much of what you say here, I must disagree with the core of the argument.
We aren't equal and opposite. While we
can use our Fire and Life power to counteract Maher's Frost and Death power, if he invests more power than we can he wins. On the other hand, if he's stingy with his protection, we could probably drive his people insane while burning the others. Let's bet on his burning through our defenses rather than the other way around, though...
You really think they'll only send justices next time? We majorly pissed them off.
This, ever-so-much. It took all our best troops to drive off a few of this god's
basic agents. We have improved our strength since then, but while he hasn't...he can send more. Even a couple dozen Justicars could probably wipe us out, especially since they know our strengths and weaknesses now...
Though inventing guns is a good idea. Battle is somewhat chaotic, chaos is innovation, and gunpowder is certainly related to fire...or we could simply magic stuff into being better than guns. Like Thorn Autocannons.
I like this idea.
Anyways...for our "We Have Too Much Mana" "problem," here's some possible solutions:
1. A mate for the Gold Dragon, or otherwise increasing our dangerous critters.
2. Relatedly, increase fertility and make children of various species mature faster.
3. Some sort of walls--I suppose that bone walls could work, but fire or something like what Limbo (D&D) is made of would be more effective (although likely requiring upkeep...). I'd suggest simple stone walls, but we can't make those. Whatever we make should have gates.
4. A modification for some species we have to make them defensive "turrets." Maybe take some Fungus Spawn and turn them into Spore Turrets, which fire explosive spore pouches. (Even if the spores died, the explosions would help.) We could later add some Flaming Spore Turrets, which fire flaming spore pouches or something...but that might be getting crazy.
5. More Mushroom Towers, ideally paired with #3 and #4.
6. Improve the Afterlife; chances are it'll be a lot more occupied soon...
7. Eternal Flame. More useful would be an everburning torch or furnace or something.
8. While a "Fountain" of Life mightn't be possible for us, enchanting a natural spring or something would.
9. Favors for Krait.
10. Helping Uristiel.