The biggest problem are the crystal dragons. With their mindscream-spell your archers are dead within seconds. I play mainly with iron ones, minions and banweavers in relation 3:1. Archers with salva-upgrade have to remain behind but then attack as well as heroes or avatar with ranged attacks.
The first nest to the west is pretty hard to conquer, there are other nests easier to accomplish.
Another tactics might be, that you ignore the crystal dragons completely and attack only the wyrms and especially the nests so that they don't respawn after a short while.
Gehabte Schmerzen, die hab ich gern!