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Defeat less than 5 hounds before killing the Houndmaster in The Horde.
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Chapter 5: The Hoard's optional boss is called The Houndmaster, and you will find a number of demon dogs in the area directly preceeding this fight. You will want to make sure you do not kill more than five of these enemies on your way to the boss fight, as well as during the boss fight. This is probably one of the hardest achievements as it could cost you a replay of the level, though it can be cheesed a little. There is a spot you can nestle yourself into during the boss fight which will immobilize all the enemies, allowing you to safely kill the boss without any dogs becoming collateral damage. See the image below for where to stand.

I would suggest using the method in this video to get through the puzzle to get into the Houndmaster's area, as this is the hardest part - the dogs will swarm you quickly and stop you from landing your void bombs to open the portal you need, as well as properly using the gate levers and fire bomb to blow up the blocked area. I had to quit the level and try a second time after too many woke up.

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