In order to win one must kill the mighty knight and the holy white dungeon heart. Being the most evil of the dungeon keepers, instead of triggering the knight arrival you'll first kill the dungeon heart then trigger the action point and win without facing the knight :). Thanks to zyzzyva for pointing this out.

0:49 I don't want to kill those wizards but I must keep them busy so they don't scare my imp 2:19 constant firebreathing is a privilege of possessed dragons and it's very efficient 3:11 dropping the imps to activate the action point in order to win the game