The Lothric Knight Sword is one of the best swords in the game and scales fantastically with your strength and dexterity, as well as if you decide to infuse to allow it to scale with faith (more on that later).
This sword requires 18 dexterity to use, which is perfect for mercenaries and has a beginner-friendly move set. In particular, you should be using these to farm for the Lothric Knight Sword.
Make sure that you pick the gold coins as your starting gift to combat this. Mercenaries start with particularly low luck which makes farming for effective weapons a pain. However, there are some early game nuggets of knowledge that you need to be aware of to use the class effectively. Like we said, the mercenary is one of the starting classes in the game, so there are no requirements that you need to fulfil in order to get your hands on it.