finally beat cave 5. ethereal goblin was such a pain. you have to keep him back with ranged units whilst destroying the enemy castle. reward is you get a spell card that gives you 100% critical (kills titans in a few seconds).
anzhobo100, you have to complete the game once. then play new game plus, and on the map screen click "cave of trial" at the bottom left corner and beat the challenges.
 "Enigmata"){kind=icon}
 "My Pet Protector"){kind=icon}
 "Elona Shooter"){kind=icon}
 "Exploit"){kind=icon}
 "Medieval Rampage 2"){kind=icon}
 "Zombie Infestation: Strain 116"){kind=icon}
 "Cube Colossus"){kind=icon}
 "Storm the House 3"){kind=icon}
 "Hedgehog Launch 2"){kind=icon}
 "Epic War 3"){kind=icon}
 "Starcom"){kind=icon}
 "me and the key"){kind=icon}
 "Cargo Bridge"){kind=icon}