For Level 14, there is a way to finish the last boss quite reliably:
build your defense in such a way, that the first hit on him will be 100+ damage (e.g. maxed crypts or dens at the entries, nothing else),
fire 5 maxed out meteors on him,
build 5 temples (and have the demon skill that gives 20mana for each new one) -> get 100 mana. as the level does not cater to temples all that much anyhow, ca. 1500-1700 gold are needed, you can raze unnessary buildings for money here..)
fire another meteor.
done.
less reliable: try to freeze (can be fired upon) or frighten him (will run around aimlessly, shortening his hide-time). a catapult den could also do enough damage in one shot to be able to kill him with 5 meteors. if you could fire all of them really instantly (and just before he cloaks), a burning eye temple may also be viable.
it is just awful, that the opponents get such massive amounts of troops in a single instant (for you, there is a cooldown, they get 10+ paladins stacked on top of each other, which kills practically everything (except bombermen)
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