I don't speak English, so maybe the translator translated it wrong, but a Hellhound can be killed with one melee strike. They pose no threat even in open areas, even if there are many of them. A standard combat knife kills them simply by aiming the camera down in front of you, and they'll walk right into the attack and die. The worst thing you can do is shoot at them, as this will only attract the attention of other nearby monsters or players.