Hellhounds are at least combat level 122, depending on where you kill them. This combat level makes them pretty strong. They definitely can not be killed easily if you are a low level not using a protection prayer. Their attack stats are pretty high and their hit points are 116, so they are powerful, have a nice hitpoint pool, and hit very accurately. However, their defensive stats are completely worthless. They have a +0 (nothing) defense stat in stab, slash, crush, magic, and range.  This means they are weak to as good as every attack you throw at them. You can easily finish them off with ranged or melee. If you are using prayer, you should just focus on getting the maximum damage per second.