The monster on cave 5 is an "almost" invisible Goblin. He can kill with just one strike and seems invincible. Keep him at bay using long range units that push back units like the mermaid as an example. I use the Mermaid, Witch, Fire Succubus and Centaur Archer as long range units along with the Goblin and Black Knight for close combat. Works for me. Hope it helps.
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