Help killing the final boss!?

Hii, so, this is my first souls like game, I don't really know what I'm doing and I can't seem to kill Radagon, I'm using a Nagakiba +25. I can dodge the first phase pretty well but I can't seem to deal with the second fase, when he keeps getting further And further away. I'm closer to killing Malenia then I am to killing him and I really want to do this before going to the DLC. Any tips? Thanks anyway 💕