Freja is one of the the 4 bosses who carry Lord Souls. The others being The Lost Sinner, The Old Iron King, and The Rotten. I'm going to assume you've already dealt with the Lost Sinner. To reach the Old Iron King, you must head through the Huntsman's Copse after paying the chick from Heide's to rotate the room for you. To reach the Rotten, you must go down the big well in Majula. You can either buy the ring from the cat that will let you survive the fall, or pay the guy you met in Earthen Peak to lower a ladder once you get him to move there. If you've already done both of these, head to the Ruined Fork Road bonfire in Shaded Woods. There's a door at the end of one of the paths that leads to Drangleic Castle, and will open once you have the 4 Lord Souls. The Royal Rat Authority is an optional boss fight, and very easy if you're quick. Try using a ranged attack to quickly take out the small rats before they can poison you. Once they're dead, the actual boss is super easy. Just try to stay underneath of him and attack his back legs.