This walkthrough comes straight from steam wich can be found here.https://steamcommunity.com/sharedfiles/filedetails/?id=19665...
Credits to Omega Ridley
Defeat less than 5 hounds before killing the Houndmaster in The Horde.
Notes: Now this one is tricky, probably one of the most difficult achievements in this game. Fortunately, there’s a way to make it completely trivial. First things first tho, you’ll want to play as Strife and not War, as the latter has a lot of range with his sword attacks, and you’ll want to focus your damage in a single spot: The Houndmaster’s face. You should also avoid equipping any Creature Cores that reflect the damage you receive towards foes.
Follow the alternate path into the Hoard found south of the main gate and make your way through the Growth bomb puzzle without killing any Hounds. Be swift, as they’ll start swarming you if you are too slow, making it difficult to progress.
With the puzzle behind you, enter the Houndmaster’s room and trigger the cutscene. As he attacks you, stay at a safe distance and unload your regular ammo into his face. Avoid Charged or Static Shots, as those have a nasty habit of dealing collateral damage and you have to keep the dogs alive at all cost. Keep it up until the Houndmaster is defeated.
If you’re having trouble, this achievement can be cheesed: As the fight starts, lead the Houndmaster to the northwestern section of the room, and stay slightly behind this crate.
The boss will stop moving and so will his Hounds, allowing you to damage him from afar without any risk of retaliation or dogs getting in the way.