Level 20 was the most difficult to get brilliant imo. I beat it by taking out the mana pool, lighthouses, then the ships, I only built stuff on the left side if I had a good amount of mana saved up. Making all the orc towers ballistas/catapults helps a lot for the divers. Terror ability if anything got to close, always have enough mana for at least one.
if you or your companion dies you can just go back to the main menu and continue so you don't have to lose any XP or your companion, it also clears the room of all zombies
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