[Endoperez]

More longbowmen. Try armies that consist mostly of Longbowmen, with just enough infantry/cavalry to keep the archers safe and to counter your enemy's archers. To help your archers, cast Arrow Fend and Wind Guide. Nature has little in the way of battlefield spells, but Panic, Vine Arrow or later Storm of Thorns, and if you're short on units Swarm. Swarm is also good to tie up enemy forces while you warm up, because those insect fly straight up to the enemy.

As for your Enchantress: do you have Astral/Earth income? You should, if you haven't known what to do with her, because searching for sites (manually or via spells) is always worth it.
If you've researched Construction, Forge Dwarven Hammer (E3, constr 4), and with it Starshine Skullcap, Crystal Coin, Earth Boots (for total +2 Astral, +1 Earth), then Rings of Wizardry and/or Sorcery for your mages. The latter two are quite expensive, being Astral 5 and 6 items IIRC, so if you can cast Forge of the Ancients. It might let your other mages to forge new stuff as well, so check them out as well.
EDIT: Oh, and she must be the Great Enchantress, who also GENERATES gems. One of the best rainbow mages there. Now, if you had used a bit more points into getting her, say, F1D2 you could make Flaming Skull and Skull Staff for +1F+1D, add in Ring of Wizardry for total of F3D4, which allows for Skullface, for further +1 to either... And there's Flame Helmet waiting you at Fire 4, if you get Forge of the Ancients up... /EDIT

If you've research Conjuration but not Construction, summon Troll King's Court. You get regenerating infantry, few Moose Knights and other nice specialty units, and an E3 mage. He can cast Blade Wind, which devastates light infantry. Blade Wind scales with level and the fatigue drop also helps a lot, so cast Summon Earth Power and/or equip the Troll King with Earth boosters, such as Earth Boots or Ring of Wizardry. Your pretender can do the same things in battle, but she shouldn't have to, and she's much too valuable. Even one death-and-recall will mean she needs both Astral boosters to forge Ring of Sorcery, and that to forge Ring of Wizardry, instead of just forging RoW.