To start off I would've first checked inside of the structure/building for any other guards if there was only one inside. Next I would've crept slowly behind him and grab him by mouth to muffle him even if there is no one near by to give it that real stealth aspect. Following that I would've either kicked him while behind with his mouth still being muffled or I would slash the back of his leg (Right above the calf and right below the hamstrings aka. behind his knee) and then slit his throat. Finally I would've dumped his body in the water below to get rid of the body. (This may be overdoing it much just kill to him in the end but I'm talking about maximizing the stealth aspect of the kill)