Great game!
Loved this game, played it through to the end, even thought all five cave trials. For anyone having trouble with trial 5, I was successful with the following setup:
Goblin Soldier, Zombie, Dwarf Engineer, Centaur Archer, Mermaid, fire Succubus, Healing Wave, Meteor Strike, Phoenix Strike and Athena Protection. All level 5, of course. If it matters, I was playing the hero "Nacht" at the time.
Enjoy!