You can beat the last cave, its simple, there are two options, knocking-back the black goblin so he doesnt attack you, or using the shield.
The first way, you put: magic mermaids,centaur archers,witches,fire succubs,magic cannon and black knight.
For spells you may use comet shot,heal,rain of arrows,phoenix strike,meteor strike and freeze trap.
Wait untill all units spawn, then advance, just let the hero in the castle, because he may die, keep your eyes on the goblin, when he animates to attack the knights, use heal and comet shot to stop his attack,when your army is on the castle, call the phoenix, use the freeze trap, and wait for the revenge waves, once the phoenix hit the castle, use meteor strike and wait the win... if you change the freeze for the shield, its the same thing, use it when you reach the castle...
hope it helps...
 "The King's League: Odyssey"){kind=icon}
 "Epic War 3"){kind=icon}
 "zombie slug invasion preview"){kind=icon}