I've killed all the bosses, except for the last one. The big rhino killed me every time with few hp and a bunch of seconds to wait. So I developed a strategy. First, try some times to beat the level killing most of enemies possible, in order to gather resources and upgrade ALL your buildings, cata and wall to the max. I don't use the tank, it's really useless.
So, if you can't beat him this way, do this: hire only an engineer and a worker. Kill the enemies approaching only with your catapult, without ending the 1st wave. Try to keep alive only ONE orc archer, then send the worker to battlefield. Wait, and win ;-)
(the worker repairing rate is higher than the archer's damage. It works with all bosses or levels, same way. You can spare the life of a weak melee unit instead of archers, if you will, he needs only to be less damaging than the worker's repairing rate).
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