I beat the 5 caves with this: centaur, mermaid, engineer, succubus goblin and frost giant. Comet shot, healing, arrows, lightning, meteor and Hercules strenght.
Try to get all your armytogether in yhe middle of field, just controling, without attacking the castle. Use goblins and giants as tanks. When you get all your ranged units together do one single massive attack with double damages, shooting your skills to control the spawns when you hear the horns. The enemy may get more than a hundred units, but your ranged will still shoot at the castle.
Cave number 4 is the mostdificult one, you may need to reatreat a few a times to kill the giant monsters.
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