Sahuagin
What is your strategy for confronting the fantasy creature Sahuagin in Dungeons & Dragons?
More info on this creature: Source 7
I would try and use sound battle formation and tactics when fighting them. Keep in mind if you are fighting them you will most likely be in water. Hopefully you are able to breath and are not sinking like a rock from your armor.
These creatures are under used by most GM's. Their society is set up in a way that allows them to be very aggressive to marine life and yet defensive enough to be almost impossible to wipe out. The fact that they are salt water life forms can possibly be used against them. If one were to use purify food/drink this could possibly be used to deter them or slow them down. Also illusions might work well as they are not highly intelligent and have, in my eyes, probably a low morale value.
In any world were these creatures exist I see them as a constant and very real threat to any sea going vessel or sea adjacent city.