Defense? A mage? 90% of fights are over after 3 turns (ice snake ~5k damage, fire rain ~4k damage + higher magic lvl3)
And casulties are non-existant (almost) as I simply resurrect all destroyed troops
As to where get emerald dragons: They're available to recruit on the great forest map. You need to go by boat as they're accesible only this way. On impossible there was 47 dragons to recruit.