[Endoperez]

There's no need to put your archers farther from the enemy if you use infantry screens, as you can do because you have already recruited lots of infantry and knights.

Nature gems could be used on some expensive spells, like Gift of Health and such, but in this case I suggest summoning. Summon several Ivy Kings and make them monthly cast Awaken Vine Ogre or Vine Men. Earth can be used up for Trolls and items; multiple Dwarven Hammers are very nice. Your Astral storage will probably dry up just from the forging lots of Rings for your other mages. If you don't have any Air 4 Crones yet, empower one comparatively youthful A3 Crone to A4, give her Dwarven Hammer and start churning out Air boosters. Winged Helmets and Bags of Wind stack, so you can get +2 Air on any of your military mages.
Water is a bit hard to use. I guess you could summon Sea Kings, conquer the seas and start spamming Falling Frost. It scales very nicely with levels in Water, so forge them both Water boosters if you can afford it (Water Bracelet, W1, Constr 6; Robe of Sea, Constr 2, W3). As for fire... perhaps just Elixirs of Life? That'd let you try all kinds of crazy stuff with your pretender without losing magic on her, and as it stops aging it can also help your Nornas.

You're in a powerful position now, so start experimenting. You could try equipping some Ivy Kings or Sleepers as thugs if you want to, to try that facet of the game as well.