To me it seems that the elemental comparison is ridiculous. I mean when your level 4 Templar Knights are getting their asses handed to them by level 1 hobbits, then there might be a slight issue.
Guys the 5th cave of trial is hard, but beatable. All u do is get full unit (succubus, witch, centaur, black knight and goblin). WShen u get to the castle, use athena on ur melee units, and hercules on ur ranged units. Then use phoenix to knock out their revenge wave. Continue hitting the castle until it falls and use ur healing potion over and over again. Keep hitting their units with ur comet.