Location:In this video guide I will show you how to defeat Royal Knight Loretta. If you keep following the Northwestern path up all the way North around the lake is where you will find the manor.
How strong to be:If you have beat Godrick and have at least put a few more points into your character you should be fine if you can avoid her attacks.
Enemy attacks to be aware of:She mostly uses spectral sword attacks that follow you if you are at a ranged distance. So try to keep a medium range and dodge basic swings and get in for an attack. She also has this nasty charged glaive attack with two sings. Once you see her wind up, run away.
What moves or abilities to use:Always use a bell summoning of your choice. I use ones with more AI to distract the boss. Plus mix in your preferred ability with your preferred weapon. You could also use a ranged class and use a lot of spells here to avoid high damage. The achievement will pop once the rewards screen comes up.